You're listening to your favorite tape on your old cassette player, and suddenly it stops working. It might not be necessary to take it to your local electronics repair shop or lament the fact that your favorite 1980's tunes are lost to you. A few basic troubleshooting steps will most likely get you up and running without having to shell out cash for an expensive repair or a new cassette deck.
Verify the Problem
Before you do anything else, verify there is a problem. Is the tape not playing? Hit stop and try again. Not powering on? Turn it off, then try turning it on again. This may sound basic, but many people will run into a one-time glitch, often caused by user error, and assume their unit is broken. Check to be sure there is a problem.
Check the Batteries
Portable cassette players, being technology from the 1980s, rely on standard AA or AAA batteries. These batteries are prone to running out of juice and sometimes doing so abruptly. Since most tape players don't have a low-battery indicator, a dead battery is almost always unexpected. Check to verify the batteries are properly inserted, then swap them out for a fresh pair. If a fresh pair of batteries doesn't fix things, then you know that the problem lies elsewhere.
Check the Settings
If the unit still won't play properly, check the batteries and the volume level. Just because you can't hear it doesn't mean it's not playing: you may have unknowingly muted or turned down the volume. If you are using headphones, swap them out for another pair to determine if the headphones are the source of the problem.
Some players have controls that change the playback speed, change the recording setup or that lock controls when not in use. Each of these controls, if unknowingly set, will cause a player to function differently and appear broken.
Check the Tape
Likewise, check the cassette tape itself. If the cassette housing is cracked or warped, it can cause problems with inserting or ejecting the tape, or make it impossible for the tape to play. If the record protection tabs have been removed or broken off, it will cause the record feature not to work. This problem can be solved by placing a piece of tape over the indentations.
Check to be sure the tape is properly spooled. If the tape is slack, or if the tape appears to be broken, creased or otherwise damaged, retest it with another tape (preferably one that you can afford to loose). Some problems are caused by a damaged tape, while others will cause new damage to the tape.
Verify that the spool hubs are turning properly and that the hubs gear-teeth are still intact. If the hub won't turn, or the capstan can't engage the hub, the tape won't feed properly and thus won't play.
Clean the Heads
Finally, if the tape still has trouble playing, the problem may be in the recording and playback heads. A head cleaner or a head demagnetizer will fix those problems and can be found at most Radioshack stores for $10 or less.