/dev is a virtual-memory-based filesystem created by the kernel and populated with the device special files which ─ among other things ─ represent the hardware in your computer. If you mount anything else at /dev while the system is running, then the original contents of /dev are no longer accessible, and by consequence, your kernel won’t find any hardware and any partitions in your system anymore.
The bottom line is that you should not be messing with things you do not understand, and blindly following some instructions you read on the internet is equally dumb.
The good news is that the problem can be fixed. Boot up from the install medium in live mode, mount your root filesystem, and edit its /etc/fstab to remove the entry that mounts your additional partition at /dev.
