Define not working...

After replacing and bleeding brakes (see below about the bleeding thing), you have to pump them up to push the pistons back into place. That is 3-4 presses on the brake lever to get things back where they should be.

If the lever is spongy but gives you some resistance, you have air in the line (most likely, since you said all the fluid drained). Bleed the brakes (easy to do, look on internets for instructions) and you should be good. Remember to pump them back up

If you have no brakes at all, did you have them before? If so, check for things like a leak in the brake line or a leak at the banjo bolt - that little flared end where the brake line goes into the rear brake caliper. Check your pads (inside the calipers - look through the little hole in the top) to see that they are 1) installed and 2) thick enough. Thick enough for now is about 2 quarters width or so. There is actually a specification for how thick, but you are looking for stuff like there is no brake pad, so you can estimate. The pad will be a composite material attached to the metal backing that the brakes are mounted to the caliper with.

Check those things, get back to us...

There is no reason that the tech would have bled the brakes with a rear tire change unless you asked them to, so they should not have loosened the bolt.