I suspect this is a Teves unit, or something similar.
After you restart the car, the system will normally recharge the
accumulator with hydraulic pressure to make the system workable.

I just serviced my Reatta system, and it came back on and recharged
perfectly.  You can normally hear the pump running as it recharges.

If it doesn't do this, you may have no brakes at all.

It will either recharge itself if you let it run a while, or it won't.
If it doesn't, take it to a qualified mechanic.  This is nothing
for a beginner to approach.