If it`s an IPOD Mini, then the chances are that you have either not connected the Molex ribbon cable properly between the Click Wheel + the logic board or you have damaged it when you were taking it apart.(Easily done !) . Take it apart and check it under a bright light (+ magnifying glass) Replacements are available on Ebay and elsewhere
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix mp3 players.