Final answer:
The best action to take after accidentally deleting a sheet is to close the document without saving it, thereby preserving any previous saves. Additionally, check for version history if the program supports autosave.
Step-by-step explanation:
If you accidentally delete a sheet that you wanted to keep, the best action to take is to close the document without saving. This action allows you to reopen the document and find the sheet in the state it was in before any changes were made during the current session, assuming you had saved your work prior to the accidental deletion. If you use a program with an autosave feature, look for the version history, which allows you to revert to earlier versions of your work.