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if you have buried wet returns or returns you otherwise can't see, they could be leaking so the water goes out as steam but not all of it comes back.

Before you do damage, get a Steam.Pro in there. Please 🙏 Mad Dog

Sorry, but you have a leak. Water doesn't just disappear like magic. If, however, you don't see any puddles around, it may be a leak in the boiler above the water line, and the water is escaping as steam up the chimney.

I Can't tell you why the autofeeder didn't try to keep up — assuming you have one and its valves are open.

I hope your plumber is a competent HVAC person, well versed in steam heat. Not all of them are…

I don't think there is a leak.

You had NO water.

a) There is a leak

b) Someone drained the boiler

If you managed to lose enough water to cause the boiler to shut down in a week — the question isn't how and why, the question is where. You have a very significant leak somewhere. It might be in the boiler on the steam side, it might be below the boiler water line, or it might be in a wet return. A broken steam line is unlikely, but it has happened.

But there's a big leak.

Don't try to run the boiler until you find it.

As to why it might not run now, @leonz may have hit it: you may have a low water cutoff with a manual reset. Which is probably a good thing, as it may have prevented you from running the boiler dry, which never good and can be catastrophic.

Speaking as a novice here:

Is there a manual reset on your low water cut off switch that you are not aware of?

Is your heating system a one pipe system or two pipe system?

Your boiler may very well have a hole in the steam chest.

Its best to call a steam licensed plumber to look at your boiler as you should not run it.