It seems to have resolved itself. Have no clue what is happening. I have my tree back on Ancestry - the one that went missing and it shows there as being linked to FTM. It also shows on the dashboard in FTM but it's not listed as one of my trees there.
I only backed up. Didn't do any other fiddling because I don't really know my way round the menus properly yet.
I will add a few things to the tree online and see what happens in FTM.
Thanks for bearing with me. Mally