I gather you have looked into the PS pump reservoir and it is empty? First, don't run the engine. The PS pump now has no lubricant and will self-destruct, in fact may already be damaged. Fix/replace the severed hose, and refill the pump with Dexron automatic transmission fluid. Best to raise the front end off the ground so there is no weight on the wheels for this next step, but....

Turn the steering wheel lock-to-lock a few times and check for fluid level in the pump. Back on the ground, you will know if the power assist is back to normal. Also listen for unusual whining or grating noise from the pump. If so, it's unlikely to last long. It may already have resulted in bits of metal distributed through the system, which would require a complete cleaning out of the whole thing, rack included.

I hope your situation is not as apocalyptic as this may sound, but it would probably pay you to have a good mechanic check on all this.