You are only allowed to authorize up to 6 devices (computers, tablets, eReaders) with the Adobe DRM infrastructure at a time.
If you have old devices you no longer use, you may have exceeded that limit.
Contact Adobe Live Chat: http://www.adobe.com/support/chat/ivrchat.html and ask them to reset all your authorizations.
You should then be able to re-authorize the devices/computers you still use.
Some of the representatives on the chat don't know what to do ~ in that case try again and hope to get one that does.
Some people have problems authorizing Overdrive.
In certain cases, you can download the Bluefire app as well and authorize and use that.
Authorizing Bluefire might even make the Overdrive app start working (???), as the authorization is really for the device, not for the app.