"Assets" Icon in Adobe Captivate 2017 no longer works. Why? Please help.

Captivate has no 'ribbon' at all, that is a MS feature, with a context sensitive set of buttons and lists. Captivate is not a MS app.

You said that it 'worked' before? That could indeed mean that the Preferences got corrupted. Other causes can been changes in the permissions for sites if you are in a corporate network. I did see a user who was not allowed to visit the website of the eLearning Brothers in his company, where the free assets can be downloaded. Did you set up an account based on your Adobe ID with eLB?

Do you run Captivate as Administrator? For the workflow described by the other user, you need Full Administrator rights, you have to close Captivate before running the CleanPreferences .... file appropriate for your OS. If you do not have Full Administrator rights I can offer you a manual solution, just tell.

If you are in the Newbie UI (you have two big buttons Library and Properties in the top right of the Big Button Bar), you don't have to care about losing custom workspaces because you have only the Classic workspace, which cannot be customized. However more customisation can be lost when resetting the Preferences which happens when you run that file. A full over view, and ways to reset custom items later on can be found in this blog post:

Captivate 8.0.1 Install? Keep your Customisation! - Captivate blog

You might try clearing the preferences for Adobe Captivate. Close down Adobe Captivate and double click on the following file:

Windows: C:\Program Files\Adobe\Adobe Captivate 2017 x64\Utils\CleanPreferencesWin.bat

Mac OS: /Applications/Adobe Captivate 2017/Utils/CleanPreferencesMac

You will lose any custom workplaces and panel undocking but if it resets the feature you can get those customizations back.