There is a sensor that activates during calls so when you put the phone to your ear, it shuts the screen off. Try using a task killer to help reduce sluggishness. If you have to many background apps running would cause the phone not to respond in some cases.
Thanks for the suggestions. I really don't have many apps running in the background and haven't added anything since this problem started. I have a task killer which might help with the sluggishness, but this is really annoying not being able to hang up a call because the screen won't come back on. Maybe I need to visit the friendly Verizon store and see what they have to say about it. So not looking forward to that encounter!