Reconstruct data after sorting error

I am new on here and I would say that I am a little more dangerous on Excel than the average user... however, I screwed up. I have been working on a spreadsheet with 28000 lines and I have gone through and resorted, researched and completed the spreadsheet over the last couple weeks. When I went to change it back to a CSV file to give back to the vendor, I realized that at some point when I sorted the list I mixed up the unique identifier (I at some point did not sort the UID only the other columns) and because I have resaved this file over itself I only have the original CSV file to go back and reimport... but dont have time to look up each of these 28000 lines again. Can someone help please? I am looking for a way to take a cell from my final spreadsheet (Spreadsheet A) then find it on the original CSV spreadsheet (Spreadsheet B) then copy the UID field from speardsheet B to a blank field in spreadsheet A.