The photos are too small for us to see any detail (images at least 1MB in file size are best) to narrow-down a diagnosis, but drastic leaf drop can happen for a variety of reasons. They include temperature stress (exposure to a warm and dry or chilly draft, such as from windows, air vents, radiators, etc.), over- or under-watering, and low ambient humidity (common indoors in winter). We have seen cane begonias do this and regrow leaves, as you mentioned, without having been able to determine the cause. You can examine the stems (since there are no leaves left to check) for evidence of pests like scale insects, but unless there has been a clear sticky residue on the plant or pot rim lately (a byproduct of scale feeding), that is not likely to be a factor. Our main suspect is root moisture that may have been too wet or too dry for a bit too long at some point.

Occasionally, exposure to enough ethylene gas (such as from a combustion source, like care exhaust getting in from a nearby door or a gas oven that isn't burning cleanly enough; rarely from lots of very ripe fruit in a nearby bowl, like apples or bananas) will trigger leaf drop, which is part of the process of normal leaf shed in autumn for deciduous trees (the ethylene production by the tree is natural in that case).

For now, all we can suggest is that the plant be kept away from any known drafts, checked for watering needs before being watered (especially now that leaves aren't evaporating water), and any drained water that collects in a saucer is emptied promptly so that the pot doesn't sit in a puddle and reabsorb that extra moisture which could drown roots. If you have the space available, transitioning the plant to growing outdoors for the summer (in a shady site) can help it put on good growth to recover and store-up energy for next winter. If the begonia hasn't been repotted in several years, removing the old potting mix (what you can without too much root damage, at least) and replacing it with fresh may help avoid any mineral buildup that may be present and damaging or stressing roots, which is common if houseplants remain in the same pot for several years without soil changes. You may want to wait until later in spring for that approach, if applicable.

If you suspect root rot (or a root-feeding pest like root mealybugs, which would be visible as white, slightly-fluffy masses clustered around the roots when you remove the pot), you may be able to salvage the plant by taking stem cuttings and rooting them, and then the old root mass can be discarded. Keep cuttings humid while they root, which might take a few weeks.

Miri