IPhone :: 4 Became Sluggish And Now Not Responding After IOS 5.1 Update

This morning my iPhone 4 became a little sluggish in unlocking, opening apps, and sliding from one screen to the next. After a couple of hours it seemed to slow down altogether with a lot of delay as I tried to operate various functions. I did notice that it seemed to open the calendar and the invitations menu on its own. Now, at the end of the day, I can't unlock it at all, nor can I operate the slider to power off. It is simply not responding to anything except the home button to switch on the screen and the sleep button to put the screen to sleep. I installed IOS 5.1 a couple of days ago.