Hi,
Today my brother and husband installed a new water heater and now there is barely any water pressure (both hot & cold) in the entire house except the basement. Anybody have any ideas?
Thanks.

Since the hot and cold are both giving you low pressure I'm going to take a guess. Ask your husband to close the main again and then re-open the main supply valve, all the way. Sounds too simple but it doesn't cost anything to do.

Thanks for your reply. Just tried that and the water pressure is still very low everywhere except for the basement sink's pressure seems fine.

Typically an older gate valve can wear out and the gate will not retrieve. I wonder if the basement flow is because of gravity and is it as good as it was? If you have old galvanized piping you may have a rust clog at a main valve or a fitting. I have been able to retrieve the gate on older gate valves by turning the water off before the gate valve and releasing the pressure and then pressing down hard as I turned the handle to retrieve the gate. What kind of distribution piping do you have in the home? How old? Have you checked filter screens and areators for debris?