Hi FiveMins, I agree, we havent had enough frost yet to freeze soil, so all that would have happened is that the tops would have been cut down !! You mention, 'large flowers' so I'm assuming you have tuberous begonias, if you place the pots on thier side, somewhere frost free and let everything dry out and totally die back you should find a tuber at the base of the plant, rub off all the compost, ideally dust with sulphur, place in a paper bag (not plastic) and store somewhere cool, frost free and dry. In spring, place tubers almost level with the compost but make sure the depression on one side of the tuber faces upwards (as this is the top) water and quite quickly, small buds will appear and from these shoots will grow !! You must do this inside and you wont be able to plant these out until the risk of frost has passed (about end of may) Make sure you grow these in good light when inside to keep the new plants bushy !! You can do this every year and keep the tubers from year to year !! Assuming of course that they are the tuberous type ??
hope this helps, best wishes Lannerman