Chances are better than even that a connection became iffy inside your probably now prehistoric cassette deck. If you're not good with a soldering gun, track down somebody that does electronic repair in town.
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On my cassette deck Nak 680 ZX I can bias my heads for proper alignment. You might need to have it serviced if your deck does not have head adjustments.
A lot of good info has been given, check connection and clean the deck.
Be careful using q-tips some use an adhesive to attach the cotton. I get my q-tips from my local electronic store, they are made for cleaning electronics, no glue. I. Also use denatured alcohol as a cleaner. I was informed years ago that regular rubbing alcohol can leave a film on the surface it is cleaning.
Other things it could be: Volume gain adjustment is faulty or input source is faulty.
Once you have cleaned and checked all your connections and problem continues send it off to get it analyzed.