The first thing to check is that the input and output cables are connected and the source(s) and speakers are working - it happens more often that you would think! (Just last week I lost a channel; it took me a good hour to determine that one of the banana plugs to the crossover had fallen out of its connector :^\ - it should have taken 30 seconds!)
The most common cause is (of course) an unreliable connection. Somewhere. But don't just go looking for one, there are too many possibilities, and it might be something else anyhow. Check the resistances and voltages, which will point to many fewer probable causes.
Post some pictures too. High resolution and good focus helps, since one can then zoom in on individual connections, solor codes, etc. The more information, the faster the diagnosis. It's difficult, but try not to make any premature changes, since they often invalidate the list of symptoms and confuse the thread.
