Strum bar working but not hitting notes

I bought Guitar Hero 3 a few days ago and played it for the first time yesterday. I used my old Xbox 360 and and old guitar we had laying around. There were no problems at all until this morning, when i saw that the strum bar didnt hit the notes. It works to scroll in the menu. The buttons work, when i press them down in a song it shows that i am pressong it down. Please help?