Happened to me once. I retried without rebooting and it worked. Probably a silly question, but: are you on the latest drivers for your GPU? When I did the upgrade to Win10 the GeForce Experience told me my drivers were up to date. I knew differently. And it took a bit of work to get the latest properly installed. If you've upgraded to Win10 and never updated your drivers you might try downloading the latest version from their website and doing a clean install. Sorry if that's not the case. That's all I've got for you on this, I'm afraid!