Something changed between the point where it worked and then did not work. Unless you can remember what that was and undo that change, you have to resort to some other method of recovery.
There is a software glitch somewhere. It could be a corrupt preference (.plist). Try booting in Safe mode. That does a Repair Disk during the startup process, so it takes longer than usual. Safe boot will then present a login screen when completed. Select Restart to reboot normally, which gets all of the functions back that Safe boot disables. See if that cures your problem.
Another possible cure is to reset PRAM (see your Help menu in Finder). You have to be very quick to get this to work. It resets a lot of things, including software switches. I have used it successfully when a Safe boot did not fix a problem.