When you upgrade to version 2.0 of the plugin it says there is an invalid header and deactivates.
You can fix this yourself by going to your "inactive" section and choosing "edit" on the plugin.
Now look in the top section and you'll see a blank line before the first */.
Remove that line.
Then choose update file.
Now go back to your inactive section and activate the plugin.
Tada! You are done!