Loud free hub suddenly become quiet
It's the pawls riding over the catches that make noise. If you hub was previously "loud" and is now quiet, something is likely keeping them from dropping energetically into the catches. I'd disassemble your hub and inspect/clean/relube.

As others (@Daniel R. Hicks) have noted, the issue is that if the pawls aren't catching, the ratcheting systems of the freehub is no longer working properly. As far as bicycle malfunctions go, this isn't the most dangerous thing that can happen as your brakes will still work, steering is unaffected, just a sudden reduction of power to the road.

Whatever is causing this (dirt or rust most likely) is likely to also have secondary effects, chiefly drag, but also the possibility of undue wear on the freehub. Five years is a decent amount of time for water and road grime to make it's way in. You probably don't need to get to your LBS today, but plan on stopping in this week. They should be able to confirm the cause in a few minutes.

Happy Riding.