The shortcut invoice bottom on the app has just disappeared. It suddenly happened by itself.

Hello there, plazafence.

Are you referring to the Invoices option visible on the QuickBooks Online (QBO) mobile app's home page?

If so, this option is still accessible, and I'm here to assist you in restoring this feature within your QuickBooks app. 

To isolate this behavior, close and restart the app. Then, clear the app's cache. The app stores cache files can sometimes cause an unusual response to the system.  In your device Settings:

Android: 

1. From your device's Settings, select Apps (varies by device).
2. Choose the QuickBooks app from the list.
3. Select Storage. 
4. Tap clear data.
   

iOS:

1. From your device's Settings, select Setting General iPhone Storage.
2. Choose an app that's taking up a lot of space. 
3. To free up the storage without deleting the documents and data for the app, select Offload App.

If the same happens, you can uninstall and reinstall the app to refresh the system. 

For more information on the features of the QuickBooks Online mobile app, refer to this article: Compare mobile app features. This resource provides a chart that compares feature availability on supported devices, including iPhone, iPad, Android Phone, and Android Tablet.

If you're referring to something different, please comment below. You can also share a screenshot. Be sure to cover any sensitive information. I'll make sure to provide the information you need.