Q: My iMac cannot connect to iCloud

I have a iPhone and a MacBook and a iMac desktop computer. All have been synchronizing iCal fine via iCloud until the past week or so when my iMac became unable to access iCloud, citing a problem:
iCal can’t log in to your “iCloud” account because your password may be incorrect."  All my other devices and apps work perfectly with my current password. And, my iPhone and MacBook are still synced accurately.

Going through my iCal preferences, iCloud will not accept my valid password. What to try next? Help please.