Hi Maddison Jones,
Thank you for contributing to the Apple Support Communities.
It sounds like you're not able to use iCloud services after updating to OS X Yosemite.
If so, first try signing out and back in to iCloud from System Preferences:
OS X Mountain Lion and later: Choose System Preferences from the Apple () menu, then click iCloud.
If you're still signed in with your previous Apple ID:
- Click Sign Out. Depending on what iCloud options are turned on, you'll be asked if you want to make copies or delete iCloud data (Contacts, Calendar, Reminders, Bookmarks, Photo Stream) from your Mac. Click “Delete from Mac”. The data will be deleted from your Mac, but not from iCloud.
- Enter your current Apple ID to sign in. The data from your iCloud account will download again to your Mac.
From:
Apple ID: What to do after you change your Apple ID - Apple Support
If you can't sign into iCloud on your Mac, try signing in on iCloud.com. If you get the same results on the iCloud site, see this article for additional troubleshooting tips:
iCloud: If you have trouble signing in to iCloud.com
Best Regards,
Jeremy