Unfortunately, mounting any filesystem on the / mount-point of a running Linux installation is fatal, unless that newly-mounted filesystem happens to contain in itself a complete Linux installation (and even then it is a bad idea).
The reason is as follows:
/ is the root mount point to which the entire filesystem tree of the OS is attached, including configuration files, pseudo-filesystems for accounting, and the binary executables of any and all commands that don't happen to be builtin commands of your shell.- If you mount anything to a mount-point where another filsystem is already attached, the previous filesystem content is shadowed by the content of the new filesystem.
That means that while the original installation is still on your hard-drive, your operating system instead sees the contents of /dev/sdc2 where it would expect the OS. This in effect makes it completely inoperative.
Since you cannot call any command anymore (remember, the shell would try to locate the executable file from a filesystem in no longer sees) your only choice is to try the "Magic SysRq keystroke":
- Press Alt+SysRq and while keeping both pressed, press in addition the sequence R E I S U B. A nice mnemonic mentioned by @TooTea for that sequence is "Reboot Even If System Utterly Broken".
This will instruct the running kernel to try to sync and shutdown the system in as orderly a way as possible (but if that doesn't work, your only choice is a hard power-off). You can then start the computer again - since you didn't modify the fstab to mount /dev/sdc2, it will boot again with the original filesystem where your OS is installed mounted as /.
For the future, the "dedicated" mount-point to temporarily attach hard-drives is /mnt.
