Why have hundreds of messages disappeared today from my Thunderbird inbox & sent folder? Because of problems at Yahoo.

Having had some internet problems on my laptop in recent days, I've just noticed that large chunks of my Thundebird inbox & sent folders have gone. Both seem to have exactly 200 messages left. Oddly, some of them reappear but there are always 200 in total remaining. My "activity manager" shows that on at least 4 occasions within the past hour chunks of 200 messages have been deleted. However, then it shows that 200 messages have been downloaded. This brought the inbox slightly more up to date (to early April) but still leaves gaps for the more recent messages and the earlier ones. As the total number of messages in my inbox amounted to less than 1000, I don't think this is a capacity problem. Any ideas??