Update Yosemite to El Captain : PANIC ERROR
First get any files you need backed up. Check your backup drive if you're not sure you have one. The Mac can be connected to another in target disk mode or you could boot to recovery and reinstall an OS onto a USB drive (even thumb drive if it's large enough) to get backup completed.
Power off and start in safe mode - hold shift when starting. If you get there, you can try the patch again, try a normal boot again and/or back up files and then try reinstalling the OS.
Boot to recovery and reinstall the OS


These are in increasing levels of time, but none of the two options presented are designed to delete user files or apps, so you would only lose files if the cause of the initial difficulties were filesystem corruption or hardware errors.