Probably because some security software (either embedded into your browser or elsewhere on your PC) is deleting all your cookies whenever you close your browser (or whenever you close Windows). Without knowing what software you've installed or modified lately, it's hard to be specific about exactly where the problem lies.
If you permanently allow cookies from the site .. you will not have to keep signing in. Can't remember when I last did!
Don't use System restore for minor things like this. You could end up much worse off than before, with loads of duplicated/copied files/folders.
I asked a similar question on another thread and Buenchico is right. I experimented by going to Tools - Internet options, then unticked 'delete browsing history on exit'. It worked, but presumably means my PC now keeps loads of cookies. I now have to discover how to delete cookies.