hi
Plesk after updating Linux from 16 to 18 with code
apt-get dist-upgrade
It crashed and seemed to be deleted from the server, but its files are still available
Well please see: Warning: Currently, Plesk does not support dist-upgrade from Ubuntu 16.04 to Ubuntu 18.04 yet. Dist-upgrade support from Ubuntu 16.04 to Ubuntu 18.04 to Plesk, will be added in a future product update. https://support.plesk.com/hc/en-us/...upgrade-procedure-on-Linux-server-with-Plesk-
So I hope you have a full plesk backup to recover your Server