WHY DID HE RESIGN???

how much time did he spend to make the last move? since it's turn-based he probably saw the fork (that's why he ""hanged" the Q) but hours or a day later he forgot it and thought he blundered?

Thought it was blitz

Actually, I think it's deeper than the Queen, and overlooked a check to save himself.  After 20.Nxf6+ Kg7 21.Nxg4 Bxg4 and now:

22.Rxd2?? Re1 is mate

22.Bxd2!! Bxd1 23.Bxd1 does NOT achieve the the skewer of the 2 Bishops after 23...Rd8 as 24.Bc3 is CHECK!.  Also, on Black's 20th move, every legal square is dark.  If he went to f8, then 24.Bb4+ and then move the Light-Squared Bishop.  White has equalized.

Therefore, Black might have to try his luck at 21...Bxc1 instead, and again, White may have thought he had to play down a pawn here, but after 22.Rxc1 Bxg4, the move 23.f3! removes the back rank threat and hits the Bishop at the same time.  The Bishop can't cover c7, and White will take on c7 next move.  This does allow 23...Bishop Somewhere 24.Rxc7 Re2 (instead of checking on e1), but then both sides have their rook on the 7th and a Bishop with 5 pawns, and a long endgame is likely.