snipping tool

a frequent cause is that you are using (runs in the background once installed) Trusteer Rapport
This blocks 'baddie' attempts to capture your screen and learn your banking details. The screen going blak, or very dense gray, is you being protected from these attacks.
If using snipping tool and your screen goes black check whether any of the tabs in your browser go to a bank or other financial institution, even if they are 'timed out'. Close those tabs. Try snipping tool again -usually works:)

Trustee Rapport is said to have a facility for turning off 'save me from screen capture', but I couldn't find it.

I'm SO glad for this last answer as I've equally been going round the houses trying to figure out why my snipping tool suddenly won't work, & was assuming it was a browser issue - but for me at least it is indeed Trusteer Rapport - so off now to figure out how to fix it, preferably without losing my security either. THANK YOU!!