HELP. I think I have lost my editing work because I accidentally saved as a layer of a smart object!?

Did you actually exit Photoshop? If yes, then your History and any saved snapshots are gone. By default, you have 50 History states. You can increase this in Preferences.

When you double-clicked the Smart Object, it opens as a new tab. Closing and saving the Smart Object saves the Smart Object in the original document. Did you save the PSD after you saved and exited the Smart Object?

Hi

When you open a smart object, it temporarily opens as a separate  .psb document that you can edit. If you just Save it then the original document that contained the smart object is updated. On closing the psb file you will be prompted to save any changes so that the original document is updated.  However:

a. You must not use Save As on the smart object (psb file) or you will create a separate document with the smart object contents and the original file that contained the smart object will not be updated.

b. Even if you have saved and closed the SO contents i.e. the psb file, you still need to save the original document or your changes will be lost on closing it.

c. You must not close the original document first before closing the smart object psb file. This is the same reason as in b. above. When you close and save the Smart object, the original file then needs to be saved or the changes will be lost

It sounds like you have done one of a-c above and your changes have not been saved. If it was a (Save As) on the smart object - then you may find the smart object saved however you named it in your temporary files directory.

Dave