Every time you save a diagram, Gliffy keeps a record of that version. At any time, you can revert to an earlier saved version of your diagram:

Go to File->Revision History*.
Select the version you would like to recover and click Okay.
If you have not saved the currently active version of the diagram, you will be asked to confirm. You can choose to Continue Working on the current unsaved diagram, or you can Open Revision to discard the active diagram and restore the earlier version.