You can try spraying with a fungicide - usual cause of this is fungal spores entering at the base of the plant, just above the compost. Remove any parts which are rotten at the base, keep dryer than usual. Once this starts though, they tend to keel over completely.

thanks bamboo, do you recommend a fungicide? i have another begonia in the same pot should i remove it ? bit wet behind the ears (thats me not the plant )

I'd certainly remove the other Begonia and pot up separately. As for the fungicide, most of the ones I would have mentioned have now been withdrawn, so its a case of going to the garden centre or somewhere like Wilkinson's and seeing what they've got on offer. Choose a systemic one if possible and make sure it doesn't say it can't be used on Begonias. It may make no difference at all though.