I have just discovered when I tried to restore a file that my Time Machine is not showing any backups. It was working fine until I upgraded to Yosemite. I have a 2012 iMac. Any advice most welcome.
Can you be more specific? Is it opening at all or is it just not showing anything going back in time?
Aesthetically, the new version can be a bit confusing. The notches on the right hand side are red (signalling something bad) despite the existence of perfect backups in time. Might you have just seen this?
Give it some time. I found that under Yosemite, TM needed time to scan the backup and display the timeline properly. As I recall, it was two or three days before it got sorted.