As it seems, the fiber link is down. There is no link light for 100BASE-FX on either side as you wrote. Note that usually, both sides' link lights go dark when one of the converters dies or one of the fibers fail.
1) One of the converters died.
Check out with a spare converter or any other device speaking 100BASE-FX. For a quick test, disconnect the 100BASE-TX side from the network (unless you've got STP active) and use an SC-SC patch cable to connect a converter to itself, it should link with itself. (Some converters require that the 100BASE-TX is linked though.) Looking at the transmitter side even through a digicam is useless due to -FX's long 1300 nm wavelength.
2) The fiber is broken
If a spare/test device links on both converters but not across the fiber, the latter is disrupted. Check the patch cables for damage or tight bends - single-mode is especially sensitive to an undercut bend radius.
Generally, it's a good idea to use pluggable transceivers in managed switches. If you use SFP transceivers with DOM (digital optical monitoring) in a managed switch you can monitor the transmitted and received power on the link.
While you're at it you might want to go for 1000BASE-LX (it looks like you've got single-mode fiber). You might also want to go for 1000BASE-BX and run two links aggregated with failover capability - there are many very affordable 3rd-party transceivers around. -BX is also a solution when one fiber has failed but can't be replaced and the other still works.