Upgrade to 10.9.3 and Facetime does not work?

I recently upgraded my MacBook Pro from Mountain Lion to Mavericks (10.9.3) and I am unable to get Facetime to work. I even tried to buy a new version of Facetime, but in doing so was told I had to get rid of the old version, but was not allowed to do ("required by OSX"). Any ideas? By the way, the error message for Facetime is "no camera available".