No Sound Output On My Amplifier, What Maybe The Problem?

Amplifiers are among the most common and easiest to understand electronic devices.  At their heart, they consist of an element, usually a transistor or (much older) a tube, that uses a small current or voltage to control a much larger one.  The output is proportional to the input, usually by a value > 1.0.
You can troubleshoot your amplifier by going through its various stages:
Power: Is it powered up?  Are the components warm?  Can you hear a hum?
Input: Are you applying a signal?  Assuming an audio amp, check the input jack wires.  Prove to yourself that the mike or guitar you are using as an input actually works.
Output: Is the amp really connected to an output device?  Prove to yourself the speaker really works.
Amplifier circuitry: If it is none of the above, it is usually the output stage transistors or tubes that are at fault.  A qualified repair shop can confirm and replace.