Cant turn monitor back on

Computer is turning off monitor after 20 minutes of inactivity, but sometimes I cant turn it back on. I move mouse, click, press every button on my keyboard, tried ctrl + alt + del, but there is no result. Then, I must restart pc.
After “googling” I found that this is “blackscreen syndrome”, and problem is avast.
Any help? I think this is big problem.
Avast is up to date.
OS is Windows 8.1.

This is not a problem with avast, but with windows and/or your hardware.

Avast blackscreen syndrome was for version 8 and Windows 8 after installing or upgrading Avast, and inmediately after reboot, and not after working or inactivity for some time. Check your screensaver and energy settings for your monitor.