I too have posted a few times about this issue. No one seems to know what the issue is and it's happening to a lot of our customers and it's getting very annoying now!
The suggestion that it's to do with 64bit OS is not true. It happens on 32bit too.
Restarting the machine will fix the problem as that restarts the retrorun.exe service, which if you stop that service manually you will be able to get back into Retrospect. But i'm very very sure you will have this problem again soon.
Do you have any AV software running? I'm thinking that may be that is causing it? I'm testing a few things at the moment to with the AV software running. If i find anything i will post back. Any other info would be great!