I have the Whirlpool NU40T61-403 hotwaterheater that is only 6 months old. I have water going into the system but none coming out It will come out at the bottom valve nice and hot but not at the top. What could the problem be?

It may have heat trap nipples that use a small ball check valve. Sometimes they get stuck closed or clogged,particularly if you are on a well. It will be on the hot water outlet.