sherwood s 6020 CP has no audio output?

Hello! I'm trying to troubleshoot my preamp, one day it was working fine and the next day it had no sound! I'm very inexperienced with this sort of thing, and this preamp is pretty old, so google wasn't much help. I've checked the fuse, cables, and plugs, and I'm about to take it apart to clean the inside. Hoping there's a simple fix that I'm missing.
 
Thank you for the reply! I don't have the right size jack to plug in headphones so I'll have to go find one tomorrow, but I did try two different speakers so I'm guessing it's internal. Any suggestions on what part might be causing the problem or a troubleshooting guide? I'm extremely inexperienced with this stuff and google wasn't much help with the troubleshooting,
 
Try switching the tape monitor switch. If necessary, move it back and forth several times
while pressing slightly, to clear any dirt on the contacts. If that does not help, try the same
thing, but with the particular switch for the input that you are using.
 
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Everything is the same as when it was working normally, except for the no output part. I can hear the electronics inside when I press the buttons and the lights all come on. I tried plugging my guitar amp into the headphones plug to see if it was an issue with the output plugs in the back but that had no sound either.