You might possibly have clicked into the miner with your mouse, leaving the cursor there and blocking it from working (known issue). If you press a key or click the cursor away mining resumes.
You can lock your window and prevent this from happening if you right-click on the top bar, then "properties" and uncheck "quick edit mode".
Thanks for the tip Dragonmike! I had the same situation before and I never though that this can be avoided by adjusting the properties! Thank you!