well, if its a nano, there should be a little switch on the top left corner of the ipod. check it right now, and if its orange, then its on lock. if so, just simply switch it off.
if that wasnt the case, it might be this: there is a setting where you type in a password so nobody can touch your ipod except you and anyone who knows your password. if you accidently made a password that you forgot, well, i dont know how to help you there.
Hopes this helps(;