Hello @OLIVIALUNSFORD -
After checking, the widget causing the problem has been identified, but we are not sure what configurations had been done prior to this - it is quite weird and probably needs certain specific pre-steps.
The good news is - there is a way for you to resolve this issue on your end.
1. In the builder canvas, select 'Card 2', and then delete it:

2. Save the app and then refresh the browser. It should look fine now.
All you have to do now is to add another Card widget as a replacement and continue building the app.
Thank you,
Tonghui