having upgraded yesterday, cannot open EM Client.
I had similar issue, upgraded yesterday (after prompt when launching the program) and all my emails and accounts are now missing! And even though I have a paid license I still cannot contact customer support in any meaningful way. REALLY NOT VERY HAPPY!
sounds like you accidentally skipped the data migration from eM Client 6. This is needed because eM Client has a new database structure.
You can import these data manually using the Menu>File>Import>Import from eM Client 6 option.