hi i'm rather new to amp repair.. just looking for some advice.. i have a behringer gmx 1200h 60 watt guitar amp head. about 2 weeks ago a friend of mine (who i'm not very with right now) accidently plugged the input into the line out on the back of the amp.. when he turned it on it let out this very loud high pitched squeel.. i ran in the room turned it off pulled the plug out hooked it up correctly, and no sound.. it turns on, and it lights up.. with the exception of a slight hum comming from the speakers i get no sound.. any help would be appreciated.. thank you in advance !

-brian