Mac Mail no longer working in OS 10.13.6

Yesterday morning I logged into my iMac and Mac Mail did not work - it has worked the day before. I was not able to connect to my gmail account and no amount of deleting, adding, or otherwise editing my account helped. I would get as far as inputting my email password and I can go no further. It just stays there with a blank gray box and no way out but to cancel. I contacted Apple support via chat and then on the phone and was advised to upgrade from OS 10.13.5 to 10.13.6 but that made no difference. Ironically, my calendar works fine and syncs to my other devices. Also, Mac Mail works fine on my Macbook Pro and my iPhone mail app is fine. So I downloaded the vastly inferior Spark email app just to have something to work with without having to resort to using gmail online. Apple support suggested it might be a hardware problem but before I pack up my heavy 27" computer and transport it by ferry to the nearest Apple store I would love to know if anyone else has had this problem and if they were able to resolve it.

Thanks!

Judy