I am storing most of my music on an external hard disk and from time to time I need to unplug this disk. After reconnecting it to my computer and starting iTunes I noticed that the individual songs stored on the external drive are now lost (they have a little exclamation mark next to them). iTunes offers me to locate the songs one by one, but I have thousands of songs. Is there a better solution to reconnect my collection in iTunes?
The easiest thing to check is the drive letter of your external disk - you can see where iTunes expects the files to be by looking at the "iTunes Music Folder Location" on the Advanced tab of the Edit -> Preferences menu.
Open Windows explorer and see what letter Windows has given the drive.
If the drive letters do not match here are Microsoft's instructions for changing the letter assigned to a drive.
If you haven't changed the location and it is still set to "My Music", it is probably due to you letting Itunes wander around your computer looking for media...
It is probably the same problem, open Itunes, right click on one of the missing files, right click and choose "get info". At the bottom of the summary tab, you will see where Itunes expects the file to be. Change the drive letter as above.