Input Isssue

I have a panasonic hdtv monitor and I have a gamecube,x-box,dvd, and vcr hooked up to the inputs but input number three doesn't seem to work when you hook something up to it it has no picture and the sound is the only thing you hear and even if you have the t.v. turned down to zero the sound still comes through as if it is turned up to level 10. when I hook up the same component to a diffeent input everything works, also the front inputs don't seem to work either. Do you think it is a problem with the t.v.? The thing is I bought a four year service plan with the t.v. when I bought it but I don't want to call them and find out I was just being ignorant to the setup.