hackthemap
An underground hydrant usually has a red sign near it that describes where it is located (like e.g. 5 meter from the sign away, 4 meters to the right). This sign looks very similar to the UI for this question in the StreetComplete app and the diameter is written at the top so you can fill it in in the app like it is written on the sign. (E.g. H100 -> diameter 100)
For more info see this page: https://wiki.openstreetmap.org/wiki/Tag:emergency%3Dfire_hydrant#Optional_tags
I'll close the note assuming this was the problem, so that the quest will reappear in the app. If there is any other problem or something is unclear, feel free to reopen and comment here :)