Many times I meet a hive, which have pupas but not larvas. It means mostly, that swarm has gone or swarm has tryed to go. I clip queen's wing at spring.
In normal swarming you can see queen cells. And normally, if queen has troubles, bees start to raise new queens. There are some cases that bees cannot raise queens.
If you don't find any queen cells, it may be that queen had become sick and it has stopped egg laying. Bees have not found the case in time and now they have no larvas.
Also if hive has some brood and no larvas, it may be that they have unmated queen. Put larva frame, and bees do not make queen cells.
It can also happen, that new queen have vanished on her mating flights, or it is squeezed when handling the hive. If you put larva frame from another hive, they start to build queen cells, queen is missing.
If the hive has been long time without brood, it may be not able to raise larvas properly. In that case give to hive a good frame of hatching brood.
Somethimes I have found a gueen, which is not bigger than a worker. It has raised from too old larva, but queen act like queen and kill it's comptetitors. That can go through queen excluder.
Worker queens are not really tru, but you get ridd of them when you give a frame of young larvas. When hive is in hopelessly queenless, tens or hundreds of workers may lay eggs. "Police bees" eat most of those eggs away, and they raise some larvas. http://www.shef.ac.uk/~taplab/flwrres.html