Hi @hcalmon! I moved your post here from that topic because I think, the two issues are not related. In the other topic the screen does not get black.
The easiest way would be a hard reset (this topic states that issues with a black screen after unlock got resolved through a hard reset), but you lose all your data ( 
).
Normally these issues are caused by an app, which messes with the unlock procedure. Maybe you can find out, when this started to happen, e.g. after you had installed some app? Then an uninstall of that app could resolve the issues. Can you think of such an app?