Hello
Ihave no water pressure and no hot water coming out of all my taps. The cold water is all good. I have a brand new water tank and it was checked and is not the problem. What could it be?

Did the problem began after the new hot water tank was installed? ALL the taps are affected? I think I have the solution but I need you to answer first. Back to you, Tom

Did the problem began after the new hot water tank was installed? ALL the taps are affected? I think I have the solution but I need you to answer first. Back to you, Tom

The problem happened after the tank was installed.all the taps are affected.

I can tell you what I think has happened and how to fix it but you're not going to like my answer, What the plumber did while installing the water heater is disturb some mineral build in the pipe walls and not it's clogging the hot water inlet ports of the cartridge in every tap in your house. You can address this in several ways.
1- Call the plumber back and have him clear the cartridges and flush the supplies, **(or**
2- If you do the job yourself the solution is to shut the water off, open up the valve and check and clean the inlet ports in the cartridge. Before you reassemble, turn the water back on for a moment to flush out the supplies. Do this on all taps affected. Good luck. Tom