Is there not a slow lane you can go in to slowly build up your stamina again? I'm a swimmer and even when I haven't been for, say, 2 weeks, I can feel it when I get back into the pool although it certainly isn't as extreme as forgetting how to swim. I thought that swim technique is similar to riding a bike in that you don't really forget how to do it if you reached an advanced enough level, it's just usually stamina that suffers a little by not going regularly. If you've genuinely forgotten how to do the strokes and such then it may be worth taking a few lessons to get that up to speed and then going back into the slow lane until your stamina's high enough to transfer to the medium and fast lanes.