Toyota Steering

Check your fuses, the power steering is in fact electric

there is a small but powerful electric motor under the dash, that is fitted via a gear system to the steering column to provide assistance to you.

the amount of assistance is controlled by a rotational sensor on the steering shaft that detects how much force you are applying and how much force is being transmitted to the wheels.

So the faster you go the less assistance you need.

the most likely cause and cheapest is the fuse

hope this helps