In Civilization V, I have iron but it won't let me build swordsmen

I built a mine on iron and at first it let me build swordsmen, but now it won't. I have not traded it away and I do not have other buildings making swordsmen.

Any idea why?

In Civ V, certain bonus resources are limited in quantity -- Iron, Coal, Aluminum, Uranium, and Horses. Unlike in Civ IV where you either had the resource, or you did not, so-called "strategic resources" are a finite quantity -- if you only have one iron mine (and that iron mine yields a measly 2 Iron) you can only have 2 swordsmen, which are iron-dependent units.

If you don't have any more of the resource, you can no longer build units that require it, and if you drop into a deficit of the resource (due to war, end of a trade agreement, etc.) all units which require that resource take a hefty combat penalty until you accrue more of it (or they die, reducing the total draw from your civ's resources).