my mac will not reboot. stuck installing latest update with 45 minutes remaining. what can i do? it has been hours

Force shutdown by pressing and holding down the Power button until the computer shuts down. Wait at least 10 seconds before rebooting. After you reboot, see if the update continues. If it doesn't then I recommend you erase the drive and reinstall macOS.

Install El Capitan or Later from Scratch

If possible backup your files.

1. Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

This will install the version of OS X you had installed.

Try shutting down the computer using the power button and then restart. This will usually 'jump start' the installation process.