I do not know if this will help you. What did you do just before this occurred? Add a new plugin or upgrade your plugins? Undo them and see.
Is yours a fresh installation or has been running well for some time?
The only time I had seen this was when my old host was overloaded. see my article on how to speed up WordPress http://bachutha.com/how-to-speed-up-wordpress-my-experience-with-slow-wordpress/. Or when Windows was carrying out its secret backdoor updates on my PC which can take hours and slow everything down. If Windows is installing updates and patches wait for it to finish. The slowness can mean that there may be a malware, spyware or virus on your PC. Does this occur on another PC?
I do not know if any of these suggestions will help you.
