My mechanic changed my brakes on a Cannondale Moterra SL 1 and when today I tried the bike it would not start not even light the screen, any hint

I had some new bettter brakes (Maven) installed a few days ago and I guess ny mechanic did something weird as he had messages like : !
Then it turned off by itself he says that he had not reinstalled the sensor properly, then did reinstall and then it turned on and then turned off by itself and cam eback on it's own he also tells me that he was shifting the mode while pedalling and that it was not working well, I know that in the past it was a current issue but thought that this was resolved... anyway he tells me that it then was working fine...

Today, a week after the brakes install for the first time I want to go for a ride and the display does ot light up nor the control on the frame... any idea if therf is a way to re-program or else to get it started? It is frustrating and would like to know if anyone had the issue and has solution

First, check that the rear wheel magnet sensor is installed and stock. If he swapped rotors something might have changed here.
After that, when someone runs a rear brake line, they have to remove the motor and battery and most likely something was not fully plugged back in and you're getting a weak signal.
Also need to make certain that the screen remained plugged in.
GL.

Try turning the bike on, using the button on the battery. They normally leave a hole in the frame with a plug in it.

If you don't have a hole in the frame. You need to jumpstart the internal controller battery. Here's a video how. It doesn't link properly. Copy and paste the link without the **** on the end into your browser.