Data suddenly shows not connected

[sp1234]

Hi, my PowerApps was working fine a few days ago but all of my data sources suddenly show not connected (shown below).

The app is still working fine and the data is still connected there, but from the creator's side, the data cannot be seen. The data I am using are Excel tables from SharePoint. 

So far I have tried:
1. Refreshing the data sources -- data shows that it is refreshing for a few hours, but it seems that nothing is happening.

2. Removing the data source and adding them back. -- PowerApps doesn't allow me to remove the data as the error below shows up.

 

3. Adding the data into PowerApps again -- newly added data does show connected, but it goes back to not connected after refreshing the app. 

May I know how to fix this? Thank you!