I submitted a ticket about this, but I was also wondering if anyone else is having this issue.
I just updated WoW Classic, and when I hit the “Play” button, the client says “The Game is running” but the play button just becomes active again, and no game appears. I’ve tried restarting my computer but that hasn’t helped. It’s weird because it’s been working all this time until just today.
Hey, Willimeena!
A UI reset (for the WoW Classic folders) and a graphics settings reset may be the best option in narrowing down the cause. This will default all graphics settings.
If this does not help, please collect a DXDIAG and post it here using the Preformatted Text button in the posting options. Just make sure the full DXDIAG contents are between the Preformatted Text tags.
Ohsnap, rapid response! 
Thanks, Ibaraius! I’ll give that a try and let you know.
Yay! The UI Reset worked. Thank you!
Glad to hear it! Thanks for the update.