You need to undo what you did to disable the MPO - edit the registry again to make it how it was before.
Alternatively, here's a link: https://nvidia.custhelp.com/app/answers/detail/a_id/5157
Basically, Nvidia had a flickering issue a couple of years ago that was resolved by disabling MPO, and then AMD had this same issue a year ago, that's why I have this on hand lol. There are 2 atttachments, one that easily disables MPO and one that easily enables it. I used it for my flickering issue when it was a thing and can confirm it works. So try using the enable one to enable the MPO again for you.