After upgrading Ubuntu from 16.10 to 17.04 on my work PC, a very strange problem is causing havoc. When I turn on my PC, everything is normal. But after Locking Computer with Ctrl+L or locking from power option, my desktop screen turns totally black, and I can just see mouse pointer. I am unable to wake computer up. I can't do anything except just restarting it.
Immediate help needed.