If re-booting your computer resulted in the exclamation marks (the !) disappearing, that suggests that iTunes can now find the files for those songs.
A test is to try playing the song in question. If the exclamation mark re-appears, then iTunes can't find the file for that song. If it plays, then it has found it.
If you still have songs with exclamation marks, use the prompt that appears when you try to play it, asking you if you wish to locate the file. Use that dialogue to navigate to the folder with the file in it, select it and click the OK button.
A clue to finding the files is as follows: once you identify the folder on your computer that has all your music in it (Usually the Music folder on your computer), look for sub-folders in there named iTunes/iTunes Media) In the iTunes Media folder:
- for a single artist album - look for a folder with that artist's name. In there is a folder for each album by that artist and in each album folder are the files for each song
- for compilation (various artist) albums - look for a folder named Compilations. In that folder will be a sub-folder for each (various artists) album and in each of those are the files for the songs
You can also use the Windows Search feature to look for files. Usually, a song's file will be named (or part-named) with the song title.