If it's a gmail account, 52.2.0 unfortunately has a problem. This will be solved soon by 52.2.1.

My email account is BT Yahoo, I have in the past carried that over to Thunderbird (as some tasks like printing and forwarding are better on Thunderbird), but on the last time of opening all the previous emails have gone, as have all but two of my folders, there may be some simple solution to this such as profiles, but I dunno!

Try this: http://kb.mozillazine.org/Recovering_a_profile_that_suddenly_disappeared

I rename my profiles (so I can tell which does what) and I cannot remember having lost a profile since I first did that.