I made some blintzes from this recipe, and froze them as indicated in the note at the end. You'll notice that the recipe only gives instructions for cooking fresh blintzes, not frozen. When it comes time to eat them, should I thaw them and then fry them, or put them in the pan while they're still frozen and just cook them for longer?
Thaw them longer and be sure they are thawed to the core before cooking; if you made them properly they will not break apart or unfold.
I wouldn't recommend warming them in the oven because most oven's lowest temp still cooks slightly and deteriorates the dish.
Once thawed FULLY cook as if it was freshly made (it, of course, will clearly have a minor difference in taste because you froze it and the best course of action would be to make less but if you have excess there's not much you can do besides add fresh ingredients regularly to refresh the old.)
If you froze them after you fried them, you can warm them up in the oven for probably 15-20 minutes. If you froze them before you fried them, you will probably do best warming them up in the oven for a few minutes and then frying them. You don't want to thaw them or they will fall apart. If you fry them for longer as you say, they will likely burn on the outside and be raw on the inside .