I am having the same problem with Mail on my imac When it opens it says messages need to be downloaded. No sidebar. I am wondering if this issue has to do with my gmail account, which I access through Mail. I have had to reenter password numerous times because the account keeps going offline. I have not had this mail issue with iphone or ipad. Thank you.
iMac 21.5″ 4K, macOS 13.3
That stinks. Gmail is sticky sometimes, and I had similar problems on an older iMac, but not the most recent model. Is your iCloud Mail turned on in system settings? System settings>Apple ID>iCloud Mail. Also, you could go into mail settings>accounts>gmail, and deselect it from your mail app (-). Then add it back in (+). If your mail app is synced with iCloud, the gmail will download from iCloud and back into your mail application.
That stinks. Gmail is sticky sometimes, and I had similar problems on an older iMac, but not the most recent model. Is your iCloud Mail turned on in system settings? System settings>Apple ID>iCloud Mail. Also, you could go into mail settings>accounts>gmail, and deselect it from your mail app (-). Then add it back in (+). If your mail app is synced with iCloud, the gmail will download from iCloud and back into your mail application.
The first step I always take in troubleshooting mail account issues is to delete the account and add it back. This generally clears up problems relating to account settings.
Can't get into Mail on imac
