Archers are only on floor 2. If anything knocked him/her off the map, it's his/her ciceda skin shedding. But, Usually when things like that are fixed with a relog because it's just an error. the monsters can still hit you, but you cannot move or attack because you're on nothing but a black screen. I've been knocked off the screen plenty of times with Ice titan and their bowling bash. I usually don't get out of it until I die, relog, teleport or get with with Bowling bash again. something to change whatever tile I'm on.
If that's not the case, then Mystra's probably right.
Remember, for cases like these, the CP has a handy little function called Reset Character.
It'll take youe character but to its spawn point.
Easy and fast.