Lost Mail after Storage shortage

My old iMac (OSX 10.13, 1 TB HDD) complained shortage of storage, when it all by itself suddenly used remaining 150GB for something I do not know. After deleting multiple files and rebooting I got some 150GB disk space cleaned. A major surprise was that when opening the Mail program, it asked to create accounts (iCloud, Gmail, Yahoo...) as if I never had those!! I can access those accounts and emails via browsers and iPhone, but I had multiple folders in my iMac HDD with unique emails stored that are not in cloud anymore. How to regain access to my accounts and local folders?