Here's what has probably happened: iTunes keeps track of your music collection by way of a (huge) database file called iTunes Library.itl, which contains links to all the locations on your hard drive where the music files are stored. If something gets disrupted, and iTunes goes looking for a file but can't find it at the old address, then it will display the little error icon you're seeing everywhere.
You've mentioned an old computer. Is that where you originally imported your CDs into iTunes? If so, you might look there in the top-level folder or directory called Music (probably visible in the sidebar of a Finder window) for a folder called iTunes. Inside, you should find both the database file, iTunes Library.itl, and a folder called iTunes Media, which should contain all your music, videos, and whatever else you've got that's associated with iTunes. To transfer a music library to a new computer, you just have to move this entire folder, including all its contents, to the new machine. Put it in the Music folder — replacing or renaming any folder with the same name that might already be there.
Now when you open iTunes for the first time, hold down the Option key while the app starts up. This will bring up a prompt to choose or create a new iTunes library. Point to the iTunes folder you've just installed. Everything should be back to normal.
Now some ifs and buts. You may quite likely have already moved the music files from the old to the new computers, even though iTunes cannot find them. You might try a Spotlight search for a single song title that you know should be there, in hopes of discovering where the whole library has gotten to. What you're really looking for is the folder called iTunes Media, with many, many folders inside it containing all your music. If you find this folder, then you have choices. You can just delete this folder if you're able to restore your library by the other method. Or: if this folder is itself inside another folder that contains the iTunes Library.itl file, then you can open iTunes while holding the Option key, as above, but point to this location. As a final resort, if you find the music files somewhere, you can re-import them into a fresh iTunes library (which will unfortunately not remember your ratings, playlists, play counts etc).
Sorry if this is confusing. Post a follow-up if you need help!