I’m not using Ubuntu 18.04 (I’m using Debian 9.6), but I recently experienced a similar issue to the one you are describing.
My particular issue wasn’t related to upgrading to a newer distribution version (i.e., Ubuntu 16.04 to Ubuntu 18.04), but rather to upgrading to a newer package version of Insync (1.5.4 to 1.5.5), which imagine happened when you upgraded your distribution version. For whatever reason, the Insync GUI crashed whenever I attempted to launch it on my machine. I could see this happening in the systray and in the terminal. And, like you, my drive folder was present on my machine but it wasn’t being synced.
I solved this temporarily by removing/purging Insync 1.5.5 and reinstalling Insync 1.5.4 via a .deb file available in the forums (https://forums.insynchq.com/t/new-insync-version-1-5-4/12724). I’ve instructed Synaptic and APT to hold version 1.5.4 until I hear some news about how others have solved this problem.