You can‘t find them and neither can LR which is of course the problem.
At least on the Mac, one trick I find that works is to take the name of the missing file, copy it, then when the dialog appears asking you to locate it, in that open/save dialog is an area where you can paste the name it the file is searched for on the Mac. Assuming it is there and found, while in that dialog, once you click on the document, LR now knows where the file resides.
If you can copy the name and search outside LR, jot down the location then do this manually, the end results are the same; LR knows where the file lives once you show it the location.
Outside of LR, search your hard disk for the photos.
Then inside of LR, when you reconnect by clicking on the ?, point to the folder where the photos are now.