Comment 1 for bug 632262

Sorry for that. That's never happened before.
To save your tasks, GTG silently backups those. You should go to ~/.local/share/gtg, where you will find a bunch of files named <some-string>.xml, <some-string>.xml.bak.0, <some-string>.xml.bak.1 ...
the <some-string>.xml is the file that contains your current tasks, while the other are backups. Just replace the right one and you should have those back. Consider that each time you restart GTG, one new backup is created and the oldest is deleted.

About the bug itself, can you please confirm that you never used the plugin before the moment you experienced the bug?
It seems (also?) an Evolution bug, since we're using their python library to access their api and in no way we should be able to break something in evolution(I'm referring to "in evolution it don't work anymore").