When in doubt Reboot!!
An obvious thing that many don't think to do is reboot the phone. These device are more computer then actual phone and like your everyday desktop rebooting can set things straight. I had the same problems as many here, what I didn't realize is that the Android synchronization program for the phone had crashed. Simple reboot sorted everything out for me.
Just saying, sometimes the answer is the simplest of solutions. Before jumping through all the hoops try it, takes less then 2 mins and at least when you install, enable or activate anything on your phone you will be sure that none of the phone systems have crashed because of other activities.
Good luck.