Have you gone looking for them, like under the bedding?

500 worms might all together be about the size of an apple. The only way to see them might be to empty them into a sieve and strain out the bedding and food as if harvesting. They are probably in the wettest part of the bin. Sometimes they like the edges near the smooth plastic. They might be all together balled up inside of some tasty food. Many vermicomposters use a chopstick or trifork to mover around material in the bin to see what is going on. We find this activity fun.

I run a worm inn and started with about a pound of worms 9 months ago. In the last few weeks I have been feeding it heavily but have seen at most 5 worms. Yesterday I emptied the whole thing in a wheelbarrow as a check - and because my toddler likes to find the worms and explore guess you just need to start them early.

When I started to pull things apart I found hundreds of worms at all layers of the compost.

This is to say I have a bin with a lot of worms in it and I really never saw any of them.

Your bin sounds new. You also didn't mention anything about off smells which is good. Keep it moist, keep feeding it, and give it time. From my reading the worms don't so much eat the food you add as they eat the microbes that eat the veg waste you add to the bin. Give the bin time for the microbe herd to build up.

Even if a lot of your worms died, they know when they are dieing and spend the rest of their lives mating and laying cocoons. The tiny worms that hatch out of them will be much more capable of adapting to your particular bin conditions, but will be nearly invisible for a while. What's even more of a silver lining is that both worms lay cocoons after mating and each cocoon is said to hatch an average of 4 worms.

If your worms died slowly enough to breed multiple times (as that's all they could do if they were dieing) then you should soon have many times more worms than you started with.

Chances are though that you really haven't lost them and they are just dispersed or hiding somewhere you didn't look. Five hundred worms are easy to see when you are holding them in your hand but it's common to not see any or maybe one or two at a time.

If they all go missing, give it more time for cocoons to hatch out. Even if no worms ever are seen again in that bin, the stuff inside will still compost so don't give up hope.

...Make sure that you bought the right kind of worms though. Some kinds can't live at room temperature. If you do see any after a day, then this is probably not the case.