When you reboot this iMac, what appears on the screen once it has finished booting.
The Finder is not supposed to "open from dock". The Finder is the Operating System, it is not something that you use to "Find" things with.
What is a "Main Bar"? Do you mean the menu bar at the top of the screen?
More information, please
IMAC 10yrs old - My main bar is gone and finder won't open from dock.