I have been doing this a pretty long time, and I think I understand your problem. I have never separated a 4 van nozzle top to bottom, but it sounds like you did. If the shaft of the sprinkler head is in tact you can replace the nozzle, usually they are not 3/4". If you can turn a nozzle on to the top of the head clockwise, righty tighty, don't cross thread, you should be o.k.. Also try to run the zone for about 5 or 10 seconds to clean out any dirt that may have entered the line or the head. If you can't thread a new nozzle on there, you will have to change the head( spray heads are usually 1/2 inch thread.
Don't try to salvage the nozzle? get a new 1 they are cheap a couple of bucks. Get the exact same one. Just do it.
Good Luck!!!
Dan