Why my iMac suddenly does not access iCloud any more?

IMac OS X Yosemite 10.10.2 Processor : 3,06 GHz Intel Core 2 Duo Memory:4 GB 1067 MHz DDR3 Initialization Disc : Macintosh HD Graphics:NVIDIA GeForce GT 130 512 MB

I have always been using iCloud but suddenly without apparent reason, the system does not access iCloud any more. I tried to uninstall it but a window says that ca't find the keys. The first sign I had was that the system stopped to file the new passwords I tried to save and forgot - or couldn't access, the old ones. Then I couldn't access my iCloud mail account through the Mail App. Them I couldn't access gmail through the Mail App. Them the Calendar stopped to sync.. If I go to iCloud on the web that is fine. My other Apple devices are accessing iCloud normally.