From time to time this issue is reported in the forum. I'm not sure where it comes from, maybe it is related to updates. That's happened to me once, too in live session of Zorin 18. These Gnome session crashes occurred quite frequently after the release of Zorin 18. Since then, several updates have been released that have reduced the frequency of the crashes.
One thing that might help is to disable third-party gnome extensions for testing if you have installed some, and to disable as many Zorin extensions as possible in the Extension Manager (those which you don't need, please keep the Zorin menu and Zorin taskbar or dash; I think Zorin Desktop Icons or User themes are also not the critical ones). However, since these extensions offer many useful features, it’s a matter of weighing the pros and cons.
Zorin 17 doesn’t crash so often as Zorin 18 and could be an option for you.
Or if you like a more stable desktop: Zorin Lite with XFCE desktop, but take a look first at this distribution (e.g. in distrosea.com) if you like that desktop.
