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 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…