Hello,
It could be your refresh rate is not set to 60Hz. You could have it set for 75Hz which is why this issue could have reoccured. You chould be able to check this by going under Right click on Desktop>Properties. There choose the Display Settings option. There you should see the screen where you select the resolution. Click Advanced and check the monitor section. There you should see the refresh rate. Check that out.
Cheers,
Mak