Why does my queen fail to win?

Why did my queen fail to win here? How can I play to win with a queen?

It failed to win because you ended up down in material.

Simply because you blundered a piece (bishop) on move 18.
And it's very hard for Queen to win against 3 standard weaker pieces.

How much is a queen worth?

@bayonetCHARGE
1 Queen is roughly (because position factor is also crucial) equal to 2 Bishops and 1 Knight.

Queen doesn't do well against rook and two minors unless the board is very open and the king is weak.