It's most likely the drive belt has gone bad or/and needs cleaning.
To replace the drive belt you need to remove the tapedeck and open it up to get the old belt out to find what its size is.
I don't know of a good place to get the belt so do a web search.
With the tape deck apart check the capstan/flywheel to see if the bushing is warn too.
For cleaning it use cotton swabs and denatured alcohol. The main things that need cleaning is the capstan, pinch roller, and head.
This site has more info to help
http://www.repairfaq.org/REPAIR/F_audiofaq2.html