One day these peppers were nice & plump, and the next day they were not. We did not have hot weather where I live. What might have happened to them?
Did the plant wilt as well? Did you open peppers to examine the inside? If the plant wilted then it's a root issue. If the plant looks okay but the peppers wilted, though the interiors look okaym it's most likely a stem issue.
This is very unlikely but how often do you water? Bells need more water then other peppers, especially if there are a lot of peppers on the plant. Yours look like peppers I've left setting on the counter too long....