My ipods greenpois0n jailbreak works but cydia wont install, how do I fix this?

i just jailbroke my ipod touch today and when i went to install cydia it will bring up a black screen that says intializing filesystems then it will go back to the lock screen and it doesnt install. How can i fix this problem, the ipod has been jailbroken before but it was backed up and the jail break removed.please help. (specs-3g- MC model