I just tried it out on LR 5.7 and it gives me an exclamation point instead of a question mark. The question mark may have been what you got in an earlier version of LR since it's what I remember seeing. But then it might just be my failing memory.

Anyway in LR 5.7 hovering over the exclamation point gives you the message "photo is missing". Clicking on it gives you a message "xxxx can't be used because the file can't be found. Do you want to locate it?"

There's a "locate" button at the bottom of the message dialog. click on that. You will get a file explorer (in Windows) dialog that you can use to navigate to the appropriate folder. In that folder, click on the appropriate file (the file name you're looking for is displayed at the top of the dialog). Double clicking on that should do the trick.

The process might be a little different on a Mac. Or on an older version of LR.

Sounds like you moved the files without consulting Lightroom. So it doesn't know where to find them. Then you tried to import them but they're already in the catalog.

Since they're in the catalog, the Library module should show you a bunch of images (if there are still previews available) or just blank spots (if the previews have expired). There should be a question mark by them. That means Lightroom doesn't know where to look for them.

Click on the question mark on one of the photos and Lightroom will give you a dialog in which you can navigate to the new folder. That will tell LR where that picture is. If that folder has other pictures that LR can't find, that process for the one picture will find all of them.

If you do that for a picture in the root folder of the new location it should be able to find all the pictures in a minimum of searches.

If you have really borked your catalog, you have two options. (1) use a backup catalog (I presume you do allow LR to backup your catalogs regularly, right?) or (2) start a new catalog and import everything. The second option will be a large chore if you have a lot of pictures.