Water was turned off during new install, now low flow and no hot water?

I have found that in turning the water off and draining the lines that when I have turned it back on I have had low water pressure. I traced the trouble to the flexible supply line to the faucet. That is the line under the sink. Turn the water off and don't drain the line like into the basement. Take the flexible line off and check it for scales. If the installer used compression fitting to the shut off then you may just have to snug them up a little more.

My motto is to always assume the problem is cheap, or at least, start cheap and move to more expensive possibilities. It's amazing how often a problem can be as simple as a closed valve, or a plug not plugged in properly.

Certainly you need to get the leak fixed, but afterwards, and prior to changing out your hot water heater, I'd check the shut-off valve on the hot water in the basement to assure that is open along with any other valves you have along the hot water line from the hot water heater to the faucet. You might also want to check the cold water valve going to the hot water heater to assure it's open.

Seems that installer did not shut down the electrical current going to the heating element before your tank had filled up with water, therefore the heating element must have partially or fully melted. Wrap old cloth around the supply valve and tie string to it to keep it tight. Keep checking the cloth to see if it is saturated while you are using the water. Maybe you can get through the weekend on this "bandage".

Get that jerk back right now and tell him what's wrong.

He should have checked all the valves and fittings before he left, he just voided his warranty

Leave it alone if you want it fixed under warranty.