Hi Nicola,
I did a thread on this a month or two ago. Hypos affect people very differently, but they CAN wipe you out for up to 24 hours, even if you get your sugars back up fairly quickly. I got rather different answers from several specialists, but the basic line seemed to be that the key is to act as fast as possible. When your levels drop too low, your body reacts by flooding itself with hormones, and it's those which then can knock you out for a long time -- the key is to stop the body doing this. I used to try to do the correct combination of slow- and fast-acting sugar/carbs, relishing the chance to select some nice chocolate etc, and also tried to keep working. but now I just completely stop what I'm doing and try to hit with loads of fast-acting sugars as soon as I get the slightest shake. Also -- experiment with different sugar-raising methods. I always used dextrose and orange juice, but they never acted fast enough. I now use jelly babies and swear by them. Hypo after-effects were wiping me out for days at a time last year, but these changes seem to work for me at least (touch wood). The key is to experiment till you find what works best for you.
Good luck!