What color is the fluid, was it changed recently before this? How was the transmission before that, weak shifts or okay? Is it making any noises now (or before it stopped)?
Mine had a flaring 1-2nd shift, dirty old fluid had clogged up the transmission valve body and there was a torn gasket internally so I dropped the pan, dropped the valve body and cleaned the whole thing in the kitchen sink, there's a post somewhere here about that. The 3HP22 transmissions are fairly simple as far as transmissions go, when I was troubleshooting mine I referred to AussieFrogs a lot, they are all about that transmission in their Peugeots. Just search 3HP22 and you'll come up with a lot of results. This page has a link to the factory service manual which I found really useful.
http://www.aussiefrogs.com/forum/techni ... anual.html
It's hard to guess what's wrong without seeing it, but sounds like it has no fluid pressure internally if nothing is happening, either something totally blocked (due to dirty fluid), blown gasket, or something just broke. I know that's not too helpful but either someone here or on AussieFrogs might know more. I eventually replaced mine with a 5-speed after I lost 2nd gear entirely.
