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Activate the amp like you normally would and note what happens. No matter what type of amp you have, something is supposed to change when you hit the on switch and turn up the volume. A lot of amps have power lights that activate when the amp is live. Also, listen for any noises the amp makes, since that could also help you determine the source of the problem. Car amps, for example, often have a green power LED light and a red "protect" light. The protect light often means a blown fuse, so you know to check the wiring when you see it. Go over all the wires, double-checking that they are plugged in correctly. If the amp doesn't activate at all when you turn it on, the problem could have to do with the power supply. Sometimes you have to contend with a loose cord, which is a very easy fix to make. Wiggle the wires, testing them to see if they are in place and cause the amp to activate at all. Car amps, for instance, often have a red power wire and a black ground wire. It also has a blue remote turn-on wire that powers the amp when you turn on your vehicle. If your amp plugs into the wall, test the power cord. Also, make sure your guitar, speakers, subwoofers, and other devices are connected to the amp if you're using them. So your amp turns on, which is a plus, but it doesn't sound right. Sound distortion can be caused by a few different issues depending on what kind of amp you have. It is often from loose wires, but it could also be your overall setup. Sometimes replacing the wires, fixing the amp's components, or changing your setup suddenly makes everything better. If you don't hear any sound but know your amp is on, the wiring is likely the culprit. Moving the wires may give you a burst of sound. You may also need to detach speakers or other devices overpowering the amp.
Look for lights indicating that the amp is turned on. Check the wiring to make sure the amp is plugged in. Test the sound quality on the amp to pick up unusual noises.