What could be wrong?

I was playing an LP on my turntable, changed LP's, cleaned the stylus with my trusty Zerodust and proceeded to play the next LP only to discover that the volume on the left channel was reduced noticeably. At first I thought that maybe one of the tubes in my preamp was dying, so I played a cd on my digital player and everything seemed fine. I checked the DIN and RCA connectors on my tonearm cable, and everything seems ok. I turned the phono preamp on and off and still the left channel volume was reduced.

Do you think that my zerodust gel cleaner somehow damaged my stylus? Or could it be that my phono preamp suddenly malfunctioned? I'm a bit baffled as to what is wrong...

Any suggestions?

Reverse the cables from the turntable to the preamp. If the volume reduction moves to the right channel, it's not the preamp.

And.....if nothing happens above (which tests turntable), then reverse from phono preamp to audio preamp to see if it is the phono preamp.

If not, then plug CD player into audio preamp where the phono preamp is currently, or plug phono preamp in where CD is to see if it is the inputs on preamp..
In all likelyhood it's not the line preamp at all because that would mean one input on it went bad and there's no electronics associated with a single input (just wires). I'd suspect the cartridge/stylus. Good luck
I've seen selector switches go bad in spots, so while there are just wires, things can go wrong with individual inputs on preamps. Take the extra minute and check everything.

You know the how it goes. The one thing you don't check is what ends up being wrong..

But yeah, I suspect the cartridge as well...
I changed the turntable connection to a different input on my linestage preamp and there was no change; the right channel was still much louder than the left. I then swapped the left/right channels from the tonearm cable to my phono preamp, and the problem switched sides. At this point, I figured the problem was either the cartridge, tonearm cable (which is relatively new), or the phono preamp. I then swapped back the left/right channels on the phono preamp, and the problem disappeared. Everything is back to normal now.

I suppose I'll just wait til the problem occurs again (if it happens again) to see if I can determine what's wrong. If it was either a cartridge, tonearm, or phono preamp issue, isn't it odd that the problem just fixed itself? Strange.

It's the CABLE. Tonearm wiring is very, very thin, and damn hard to make connections with.
You "new" wire is probably loose at the cart/wire interface, where the wire is clamped into the clips that slip onto the cart pins. I would inspect those clips very carfully... I bet the wiring on one clip is not done very well.
Sometimes cables oxidize, and just by unplugging and replugging, you have cleaned the connection.
Another issue could be the cantilever on your cartridge is out of position. This could be caused either by mishandling or defect in the cartridge.

Get a pair of magnifying lenses and look at the cartridge straight on from the front. The cantilever should be evenly positioned from left to right in the cartridge body with no tilt on the stylus. If it appears off, this could be generating the volume difference since the far end of the cantilever will be out of position inside the body of the cartridge.

If this is the case, sometimes removing the stylus assembly and reinserting (if removable) will correct the problem. Otherwise you might need a new stylus assembly.

Also check the weight and anti-skate settings on your tonearm, and double check the overall alignment.
Are you using the arm lift to drop the stylus into the zerodust?

If you are hand holding the zerodust and pushing it up on the stylus it will cause the cantiliever to be stressed side to side and can damage the stylus. One of my local dealers stopped carrying these because of his customers damaging their stylus.
I had this problem with a Sumiko Bluepoint Special and cured it by gyrating the cantelever around in a very small circle. I don't know why this worked but as a last resort I was just screwing around with it trying to find sht was wrong and got lucky. It has croped up once again with the same cart and was cured the same way.
Thanks for everyone's responses. I figured out what was wrong. It appears the connection from the cartridge to the tonearm cable was the culprit. When I originally attached the very thin tonearm leads to the pins on the cartridge, I just slipped them on; they are a bit on the loose side I'll be the first to admit. Obviously the connection wasn't very good, causing the sound from the left channel to fade out. All I did to fix the problem was just wiggle the connections and everything was ok.

So how do I prevent this in the future? I was thinking of using a pair of needle nose pliers and just crimping the connection to make it fit tighter. I'll only do this as a last resort because the cartridge pins are SO small and I don't really want to damage anything. Are there any other options for me? Any suggestions would be appreciated!

Michael Fremer suggests using a toothpick to push into loose cartridge clips and then GENTLY squeezing the clips with needle nose pliers to close them down abit. You will have to remove them from the cart pins first, I know it seems obvious, but .............. Good luck.
You fixed it by cleaning the connections. Its a good idea to unplug and plug everything once in a while to clean.
The old toothpick trick works perfectly. It prevents you from squeezing the clip too much and flattening it out. A round wooden toothpick works best, just the shape you want and it "gives" a little.

Don't use needlenose pliers unless you also use a light touch. Finger pressure is often enough IME. Like any electrical connection, clean surfaces and a snug fit do work best.
After the above, contact cleaner followed by an enhancer will ensure a trouble free and sonically superior future.