Error after about 3 weeks - contains no items

After working perfectly for 3 weeks, suddenly my primary mail account is showing “This folder contains no items and no operations can be performed”. This is on the root directory as I cannot expand or even see any folders / etc under this account.

Further to this, there is a red triangle with ! which says:

Connection Failed

An attempt to connect to email@address.com failed. This could be caused by temporary server unavailability or incorrect settings. Please check the settings or click on the warning symbol to try connecting again.

I have also removed and readded the account and also restarted, to no avail…