Are you asking about photosynths from:
1) the original desktop application or the 2014 Technical Preview or
2) panoramas from ICE, Photoshop, Windows 8.1 Camera app, or Photosynth's mobile panorama apps?
A different way to ask is, "Are you asking about a panorama that you've deleted from your own device or getting back something something that you uploaded and then deleted?".
The short answer to 'undeleting' synths and panos that were uploaded to Photosynth.net and then deleted from storage here is, 'No.', as far as I know.
If you still have the local copy you can, of course, re-upload it, unless you've deleted that as well.