We use Orbenin eye ointment for this, applied at least once a day, twice if you can. Open the eyes right up so the ointment gets in well under both top and bottom eyelids. Your ewe will need a bit of extra attention until she can see again, to make sure she is feeding enough and finding the water.
Because both eyes are completely opaque you may find she takes a while to heal up, and there could even be a bit of corneal damage, although the only sheep we had that bad did eventually get rid of the scars.
This is contagious so watch your other sheep and at the first sign of redness in the eyes or opacity start treatment with the Orbenin (which isn't cheap
) Your vet may suggest a different treatment as may others on here - hopefully someone knows something less expensive but as effective.
Incidentally, it won't have developed overnight, but you have perhaps missed the early signs.