I'll help you resolve the issue with your QuickBooks Online app that no longer access to suppliers.
It is an unusual behavior of your app. Thus, I recommend resetting your app data to refresh your connection.
Here's how:
For Android devices
Go to the More Options.
Select Settings.
Click Refresh Data.
Then select Yes.
For IOS
Go to the Menu ☰.
Select Help & Feedback.
Click Refresh Data.
Then select Refresh.
If you still have the same issue after refreshing the data, you can uninstall and reinstall the app. Then restart your phone. Refer to this article for the steps: uninstalling and reinstalling the app.