DVD Help

I lent a DVD video disc to someone. After I got it back, it will no longer play on my home or office computer DVD drives. The drives don't recognize a disc in there. My DVD burner drive will play it, but It skips from time to time. It looks clean. Any suggestions? It's an educational DVD that cost close to $300 and I'm not anxious to have to replace it.
Can you return it for a replacement? Next time loan a copy.
I rent DVD's and have had this problem from time to time with rented discs. About a month ago I came across some recommendations for the Aleratec DVD/CD Disc Repair Plus Kit in USA Today. I got one a couple of weeks ago from Amazon.com for about $27.00. I haven't had a chance to try it yet, but from all the reviews I've seen, it promises to be a good buy.