Problems with "undo" (cmd + z)

Hi there!
I was copy-pasting some stuff in a text box in OmniGraffle from another text box, but at some point I used a wrong shortcut (I was using “Shift + opt + cmd + V” for pasting but apparently I pressed some other key) and my text box disappeared. I used cmd+z several times but with no luck in making my text-box appear again!
I tried also the “Revert to” option, but with no success as the document did not autosave in the process.
Could someone help me please?
Thanks in advance