Hey Kat!
if the amp is brand new...take it back to the store and ask for a new one....
did you check the guitar-cable, try various ones.....sorry for asking something stupid, but i had a lot of customers asking me about problems, and thats one of my first answers!
if not....tell me the type, brand, everything you got on it (fx-loop, whatever)...i'll write you a "first-check-list" what you can try before we have to open and check what could be wrong....(don't forget....110Volts inside....tube-amps even more....around 300 - 500 Volts....be carefull)
i'll help you as much as possible....(its my day-job)
http://www.bassline.ch/paco_casanovas.htm
all the best
Paco