Press Ctrl+Z.
If it is too late to undo, type ^$ for target in the Advanced Display Filter field and check Regular Expression. Select all filtered segments and change their status to Not Translated. This handles empty target segments. If you have a lot of partially translated (draft) segments, it can be easier to re-import your file and pre-translate it from TM. Even though all the segments, including empty and draft ones, get confirmed, they are not added to your TM, so you can safely use Pre-translate with no risk for unedited fuzzy matches to appear as 100% matches. 

Thank you, Stepan

Hi Stephan, thanks for the answer.  After I've filtered the segments, how do I change their status?

Hi tzy_2d00_jinchow,

First select all segments, so you'll see everything in the Editor with a yellow background.

Then right-click on any place of the left hand side column (the one with segment numbers) and select Change Segment Status.

Then select the status you wish for those segments:

Thank you so much!!  I've managed to do it!  I wasn't sure how to select all but finally managed.  Thanks again for the help.