You have three possible problems: the amp, the speaker, and the speaker cord. Does the amp work with a different speaker? Does the speaker work with a different amp? Does the speaker cord work with a different speaker or amp?
First thing I'd do is try plugging the cord into the left and right inputs on the speaker. Also try the External Speaker jack on the amp. Try all combinations. My reasoning here is that the mono in jack is a switching jack, and something could be wrong there. The left and right inputs should be straight to the speakers. Of course, I could be wrong. If possible, do this with a known good cord.
If none of that works, you'll need to try something else. Do you have an ohmmeter? If so, test the cord for continuity and for shorts.
Okay, I'm going to stop before I go through the entire troubleshooting scenario. I'll just ask, do you have an ohmmeter? Do you have known good amp, speaker and cord? If not, do you have friends with any of these?
