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You may have, instead of de-selecting and then Deleting, have not de-selected and have actually Deleted those photos.
Deletion should be done with care.
What were you working in, the Export screen?
If the Export screen begins with a listing of files to choose to Export, look in your computer's
for all those photos. Hopefully that's where they are, you have but to select them all there, right-click on the selected group of deleted photos, and choose 'Restore', to put them back into your filesystem and back into usable status. Try that soon, not later. The longer this is put off, the more likely it is that these photos will not be recoverable this way.
I hope it works out well this way. Let me know. I stand ready to advise you further.