My computer wont recognize my ipod touch!?

i just got an ipod touch 4th gen 8gb and when i first plugged it in to my computer it worked fine but now when i plug it in my computer says 'usb drive not recognizable' and it doesn't show up on itunes if anyone has any advice i would really help it, cause this is my first ipod and it still has no songs on it cause the computer wont recognize it!