How can deauthorize computers that are no longer working?

Use the "Deauthorize All" option and then authorize any computers (iPhones/iPads are not affected) that you still have and want to use.

See the following from Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

If you need to deauthorize a computer that you don't have access to

To deauthorize a computer that you don't have access to, you have to deauthorize all of your computers. Then authorize each computer that you have again. You can deauthorize all computers only once a year.

1. Open the Apple Music app or Apple TV app.
2. Choose Account > Account Settings.
3. Sign in with your Apple ID and password. Forgot your Apple ID?
4. At the top of your Account Information page, go to the Apple ID Summary section.
5. Next to Computer Authorizations, click Deauthorize All.
6. Authorize each computer that you use again.