El capitan upgrade takes forever (>48 hours)
The chance that something useful is happening is low. If you are comfortable with terminal or have a second Mac I would power off the Mac and then examine the /var/log/install.log and /var/log/system.log to see what has transpired to get stuck.

Restarting the upgrade or reinstalling the OS from Recovery HD should be harmless and would very likely succeed.

You could also power off the Mac and connect it to another in Target Disk Mode and get a backup if you have none / don't trust the backup / it's not current. That way you can erase and install and migrate your user, settings and apps back using Migration Assistant or do it by hand.

Unless you can ssh in or use remote desktop to get the files mentioned above, you have few options but to pull the plug and pick up the pieces.