What is your configuration? As in... how big is the brood chamber? are you using a queen excluder? any supers on top of the brood chamber? deeps, mediums, shallows, or a combination?
That information may or may not give me a clue... but I'm not very experienced either.
Could the bees have run out of space for laying brood?
Are you sure one of them has no brood at all?
Here's my take, but keep in mind I'm a newbie too:
If you've only got one frame open for brood laying, I would expand the brood chamber, especially now that the coldest part of winter is over. If you've got lots of bees, either the queen is just recently gone, or is doing fine. I would check it again in a week and see if there are fewer bees and still no brood. Queens don't lay as much brood in the winter as in the warm seasons.