First of all, is the car an XJ8 or XJ6? There is no 1999 XJ6 so it has to be one or the other. Second, was he messing with the ABS Wheel Speed Sensor while changing the breaks? it might be triggering the Traction Control to kick in while driving? Third, any lights on the dash? does this happen at a certain speed or engine speed rpm? Are you driving aggressively at stops to trigger the ASC/Trac?

yes it is an XJ6 1999 there's no lights on the dash, there is interior power and no I'm not aggressive drive I'm very careful while driving, and I did notice the passenger head light working. When I turn to start the car up nothing happens. The power cut off out of the blue while I was driving. Just seems odd this happened after getting my brakes done.

I suspect 2 things, ...a possible key module reader failure...or bad battery cable.
Does the engine turn over or are you not getting anything when inserting the key into the ignition? Do you have 2 keys and have you tried them both?
Have you checked the battery, how old is it? Is there any corrosion on it or the cables. Corrosion is a murder suspect for these cars and will kill their ability to function properly.

I have just the one key, the battery is in good condition, I get nothing when I turn the key. I do have interior lights and windows that function just fine fine, but nothing happens when I turn the key to start the car. The dash does not turn on either.

I was thinking maybe one of the fuses???? There's tons of them, and maybe the relays???

check out all the fuses and switch around some main relays also check the grounds. And by the way your car isn't a '99 i checked your photo album it is a XJ6 X300 model ('94-'97).