You cannot yourself deauthorize a computer that's not working or to which you no longer have access. If you really feel you need to deauthorize that system now - it won't do much other than getting you back that authorization to use on another computer if you're already approaching the limit - you can try contacting the iTunes Music Store customer support department through the form at the bottom of
their web page and explaining the problem to them. They might be willing to clear your authorizations for you.
As to recovering the apps and music, applications can be downloaded again. If you didn't make a backup of your apps (something you absolutely should be doing on a regular basis), go through the purchase process as normal and it should allow you to download the apps again without being charged. Music, videos and movies are not the same, though; they cannot be downloaded more than once per purchase. You need to restore them from your backup or from an iPod, if you have one. If you have neither, you can try contacting
iTunes Store Customer Service and explaining what happened. Reportedly in some cases Apple has allowed redownloading on a one-time basis, but don't count on it.
