Try resetting it: http://support.apple.com/kb/HE36. After resetting it, you will have to go to Settings>iCloud and tap Delete Account, then sign back in with your new password. When you delete the account it only deletes the account and any synced data from your device, not from iCloud. Provided you are signing back into the same account and not changing to a different account, your data will be synced back when you sign back in.
If you have any photo stream photos that are over 30 days old that you want to save, save them to your camera roll before delete the account. To do this, open you my photo stream album tap Edit, tap all the photos you want to save, tap Share, tap Save to Camera Roll.