Hi Bob, how did you upgrade? Did you do a dist-upgrade?
In the /var/www/emoncms/ folder please do;
git status
git describe
There is another thread here Mariadb cannot connect after dist-upgrade does that help at all?
The solution was in answer 4 Mariadb cannot connect after dist-upgrade - #4 by borpin
ProtectHome=false
[edit]
The correct way to amend the service file is outlined here
https://github.com/emoncms/emoncms/issues/1202