The exclamation mark appears because iTunes can no longer find the file for that specific title. Remember, music is not "in iTunes". All iTunes does is catalogue the music you add to it and remember where the file for each track is located (on your computer).
When iTunes can't find the file, it is usually because the file itself (or the path to it) has been renamed, moved or deleted, either by another programme or by the user. It might also be because the file is on an external drive that is no longer connected to iTunes (however, since you say that this is affecting tracks from your CDs, that particular scenario seems unlikely).
The first thing to do is work out where the file should be and then check to see if it's there. If it is, then work out why iTunes can't see it (i.e. one of the reasons above; renamed or moved) and rectify the situation. Be careful though, if the issue is the file path has been changed, do not simply rename the path as this will affect any other track in that path. If the file is not where you expect it to be, check other locations on your computer.
Use Windows Explorer (the file manager), to look in the expected location for the file, as follows:
- CDs by a single artist are usually in Music/iTunes/iTunes Media/<artist name>/<album title>/<track title>.
- CDs that are a various artists compilation will be in Music/iTunes/iTunes Media/Compilations/<album title>/<track title>
(Sometimes in older installation, files may be in the iTunes Music folder instead of iTunes Media)
So if it's only one track missing, begin by looking in the folder where the other tracks on the album are located.
However, if the track features a guest artist, you may find that the "artist name" folder is both the main artist and the guest artist. For example, songs from the Nelly Furtado album Loose:
- track by Nelly Furtado will be in Music/iTunes/iTunes Media/Nelly Furtado/Loose/<track title>
- track by Nelly Furtado Feat. Juanes will be in Music/Nelly Furtado Feat. Juanes/Loose/<track title>
Note the difference.
Another way to find missing files is that usually the filename will be closely related to the song title. For example, the Nelly Furtado track Say It Right, which is track 8 on the album Loose, will be named 08 Say It Right.m4a. (the file extension - the m4a bit, depends on which format you copied the file to. iTunes default is m4a. So you can search for Say It Right in Windows Explorer)
Once you find the file for a missing track, see if you can work out why it went missing. Once you know, it may help prevent this happening in the future.
To add it back to your iTunes Library, make a note of its location and in iTunes use the "Locate track" option that you mentioned, to navigate to that location, select the file and click the OK button.
If you cannot find the original file, and you do not have a backup of it, or the CD that it came from, one option is to simply buy it as a single track in digital form. Tracks are usually available in the iTunes Store, or stores such as Amazon and Bandcamp.
Regrettably, I personally can no longer recommend buying MP3s from Amazon as their procedure is so complicated that it's off-putting (I wish I was joking). So if you do not already buy MP3s from them, I suggest that instead, use the iTunes Store or Bandcamp or any other online store. Bandcamp's catalogue is not exhaustive, so look at other stores too.