Since we don't know what you've done thus far in entirety, I would suggest backing up any data and reinstalling from scratch.
Booting with the "C" key will tell the computer to boot off a disc - but if you've got a Mac mini server, then there's no disc drive. To boot off an external, hold down the "Option" key at boot and choose the drive to start from.
iMac 27" 3.4GHz Core i7, 20GB RAM, 256GB SSD • 27" LED Cinema Display
MacBook Pro 13" 2.8GHz Core i7, 16GB RAM, 120GB SSD