Sudden loss in hot water pressure?

Sounds like debris has broken loose in the main hot water line and is restricting flow. If it is just the hot water line you can try pulling the stems or cartridges from each faucet and testing pressure and volume at each point. If there is no change, you are going to have to go back and check the water heater. Hook a garden hose to the drain on the water heater and check the pressure and volume there. If that's OK, then you need to start braking down the hot water lines to find the stoppage. This is a major job for anyone to attempt. Good luck.

You don't say in your question whether the HWS is a Mains Pressure type or a Gravity feed type. If it's a Mains pressure type the water pressure filling the storage tank from your Mains from an external Water Company supply, should be basically the same as the water pressure in the hot water outlets and pipe work throughout your installation and at that you should have a greater pressure at your taps etc than you do if you have an elevated Gravity Feed HWS. Gravity fed HWS just rely on the height of the HWS above the hot taps in basically a vertical difference. The hotwater in these type of systems is normally at a much lower pressure as it just relies on gravity to generate the hot piping pressure. If that's your system and as said by another respondant the cold water filler tank usually attached to the HWS has a stop cock which turns the available water off and in that tank is a ball float valve that fills the tank as water is used through the HW supply. Normally the problem occurs as said if the stop cock tap wasn't fully turned on completely after the replacement of the HWS The Ball float valve can also be "Sticky" in the fact that it stops fresh water entering the smaller filling tank later as the ball valve may not drop as the HW is being used therefore the pressure becomes lower until the cold water is allowed into the main storage tank by the float valve to top it up. Another problem for "Some" water users that due to the fact they are very late in paying or even not paying their bills for water usage etc; the Water Company instal a "Restrictor washer " in their side of the "meter" etc so as just to permit a small flow of water through your system for health reasons which in turn seriously lowers the mains pressure. There is another possibility that you have an external stop tap away from the HWS somewhere to allow the replacement of "tap washers" without having to turn the water off at the mains or the stop cock adjacent to the HWS. Another thing sometimes occurs if the Water Company have worked on the Water mains even in your street or area and have turned their stop valve off but haven't fully opened it again when they completed the work they were doing at the time. The only other suggestion is that if it's a Mains Pressure type which has a water pressure control valve, again adjacent to the HWS; it may have been altered at the installation of your new HWS and is availed of to lower the inlet pressure coming into the HWS due to the fact that if the actual pressure is too high it can stress the internal water tank in some cases where it will force a damaging leak. The last suggestion is that some forein body or blockage has occured when the pipes were disconnected at the time.. . .

I had a chunk of ice pass down a pipe & knock off a huge amount debris & literally clogged faucets & toilet water supplies. Pull your aerators & run the faucet....be a detective!

Check the screens for debris, single handle kitchen faucets have a screen where the handle meets the hose- there's a screw on connection there.

dip tube in water heater time to change it

Don't leave them on overnight.