Why wont my ipod touch connect with itunes after my sister used my itunes for her iphone?

my ipod was working fine with my itunes until my sister got a new iphone and used my itunes for her iphone. she said all she did was install safari and something else that it said she had to and update itunes. then she said she signed into her itunes account...but now my ipod touch wont connect with itunes at all..it still makes the noise when i plug it in but wont show up anywhere. i really want to be able to use my ipod again. and i called apple and its not under warranty anymore so they said if i want help i have to pay like 30 dolllars just to figure out a problem that im sure is simple to fix but ive tried everything that i can find on the internet. please help if you kow what might work.