Well, it would be surprising if the up/down actuator motors (or their position potentiometers) on both mirrors would be broken down
at the same time. It could be :
a) a broken down knob controlling the mirrors (the thing itself always gives a flimsy feeling, like a worn out gearbox levier..)
b) a bad or broken contact in the wiring between this knob and the Control Unit taking care of the mirror positions, assuming this knob
has, besides its other functions (normal=up, left mirror control, right mirror control and mirrors retracted) 6 wires going to
the Control Unit : common updown line-up line-down line-common leftright line-left line-right line ; the lines in BOLD
might have a problem
c) a broken down Control Unit (something wrong with the output "common updown" towards the knob or with the inputs
"up" and "down") from the knob
d) a fuse, but that would probably block a complete mirror operation on one side, and not create the problem you have.
Get a good car-electrician or electronician have a look at the knob and see if he can sort out the 6 wires and bypass
the internal up or down contact in the knob, setting the knob beforehand to e.g. "right hand mirror adjustment" and
check if the mirror now moves up or down
I would go for a replacement of the knob (try Eurospares or E-bay)