Fernanda,
I have used FireFox for years and this is one of the main reasons why. I have had this happen as well as accidently losing text in other locations on a page. I don't know which browser you use, but if it is FireFox there is a fix for losing data like this. There is a form recovery add on for FireFox called Lazarus. It has saved me many times!
It works in Moodle 1.9xx a little more intuitively than in Moodle 2.xx but it does work in both. When you lose the block, create another one. In Moodle 1.9, you can then right click in the text box and select "Recover Form" and it will magically put all the text, including the title back.
In Moodle 2.xx if you click in the text frame it won't work. The trick is to right click just outside the text frame, then select "Recover Form" and again it will be restored.
Hope this helps in the future,
AL