There could be a simple solution too. I had same issue, screen will go black once locked (or auto lock). Phone will remain on, but nothing will be visible. Removing battery and re-inserting only will bring display back.
Now it’s fixed. Here is the solution. Check your brightness level, if it is reduced to zero, increase a bit and set it. Lock your phone and unlock - you see no more display black issue.
P.S: This is applicle only for display changed mobiles