The exclamation marks mean that iTunes can't find the files.
First check that your music files are on your Pc, usually they will be,
Right click on one of the tracks with an exclamation mark and select get info.
Answer "No" when iTunes asks if you want to locate the file.
Get Info should be displayed. Click on the Summary tab if not already there.
You should see the location where iTunes thinks the file is located.
Does this correspond to where the files actually are?
The next thing to check is the loaction of the iTunes Media folder.
Is it correct? Some time iTunes can default back to a default location for the media folder when you actually have it somewhere else.
Changing a drive letter is another possibility.
This happens if the file is no longer where iTunes expects to find it. Possible causes are that you or some third party tool has moved, renamed or deleted the file, or that the drive it lives on has had a change of drive letter. It is also possible that iTunes has changed from expecting the files to be in the pre-iTunes 9 layout to post-iTunes 9 layout,or vice-versa, and so is looking in slightly the wrong place.
Select a track with an exclamation mark, use Ctrl-I to get info, then cancel when asked to try to locate the track. Look on the summary tab for the location that iTunes thinks the file should be. Now take a look around your hard drive(s). Hopefully you can locate the track in question. If a section of your library has simply been moved, or a drive letter has changed, it should be possible to reverse the actions.
Alternatively, as long as you can find a location holding the missing files, then you should be able to use my FindTracks script to reconnect them to iTunes .