I had a similar problem last spring. I had a P/S light and the power steering wasn't working at all. I checked the fluid. It was only a little bit low. I topped it off. The P/S light went off and it worked fine for several days. Then it happened again. I checked the fluid.. It was still full. HHHmmmm?
So I took it to my tech friends. I've known them personally for years.very capable, very good with any import. Couldn't find anything wrong. Ran all kinds of tests. It is tied to the ABS system somehow. Sharing some common circuitry..the pump is electric.
Anyway, when we started the car it was working again. And has been working fine ever since...A mystery???
I know this doesn't help.. but maybe your problem will end up in a similar fashion..oh, they didn't charge me anything...
anyway, I would have to go with blackdelsol's suggestion...oh, check the fuse-since it is electric.