Obviously make sure your water supply is turned on. Did he pull the unit out to uninstall it, or did he do it in place through the hole like I just did on someone's today? If he went in from the front, there's not much way he did anything other than knock something off the board. What you should do is select hot wash, and cold wash and see if you get anything off either. Make sure the shutoff's are on at the wall, and if they are call him back out there and make him fix it. There's no way anything broke from changing a gasket, however it's easy to knock a wire off. And if you paid to have a gasket changed it's not right for me to walk you through tearing the machine down to fix something he broke. Thanks!
Not imparticular, but if he was in there banging around, the control board is upfront with all wiring on it. You need to either have him come out, or remove the 2 screws in the top panel and take it off , and check each connector 1 by 1 on the board. Most likely it's just a loose connection. Easy fix, but I'm a principle guy, I would wait a week to have him fix it rather than just fix it myself if he broke it. Thanks!