What is the computer doing when the monitor goes blank? How often does it happen? Have you installed any new software lately? Did this just start happening or has it been happening for a long time?
In addition to @strollin’s question about whether you have had any driver updates recently, I’m curious: when you say it has been ‘fine for years’, how long have you had this monitor for?
What sort of connection are you using, e.g VGA, DVI, HDMI? And does the power indicator light on the monitor change colour or go off whne it goes blank?
How frequently is it powering off?
The issue could be one of a few different things so if you could answer our questions it will help narrow the issue down
If you go Start > Settings > Updates and Security > View update history it will show any recent updates.
If the monitor takes VGA and you have had it for several few years, there’s a chance that it is simply getting old and unreliable. It might just need a new VGA cable.
It might be worth reinstalling your video driver to see if this helps.