could be many things, first the obvious. volume, gain etc on the amp should be up. just for testing purposes turn evey dial on the amp so that it points to the middle of its range of motion. then make sure the guitar has the volume turned up. Also make sure the cord is plugged into the right inlet on the amp. A lot have a headphone and a effect loop send and recieve if you plug into one of them by accident you will get unpredictable results.
after doing all this
If it has a pickup selector check to see if it makes sound in any different positions.
Take it from here and post if this dosent fix it.
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May be btw thanx