That tends to happen when you have a conflict of some sort....
=> Did you add any new plugins lately?
=> Did you move to a different theme?
=> Did you mess with any code in Appearance > Editor?
=> Are the theme and plugins you are using tested up to the most recent version of WordPress?
=> Is your installation the most recent version of WordPress?
I have an article in GoDaddy's Garage Blog that can help you troubleshoot conflicts.
"Worry is a misuse of imagination." ~Unknown