I have an ASUS FX502 GL502VML laptop.  Running windows 10 pro. 

If I leave the laptop it goes to sleep (I think this is the term), the screen goes black.  When i hit a key or wiggle the mouse the screen goes first a light black for a few seconds (so the screen is definitely doing something) and then the login screen appears.

Lately, and its getting more and more common, the screen goes to the light black bit but never ever carries on to the login screen.  The lights on the keyboard are on, the fan is going, I can see the battery and HD lights etc, but it never ever comes back to the login screen and I have to reboot.

Can anyone give me pointers as to what to look into?  I'm not hugely techy.  I've tried googling but haven't come up with much, but I am probably not using the correct terms.

Thanks