I was playing around with my Raspberry Pi and I accidentally bricked it! How can I restore it back to the factory defaults?
You can reset the device to factory defaults by reflashing the SD card.
You can find advice on how to do this here: How do I install an OS image onto an SD card?.
Bricking a device usually refers to getting the ROM into a corrupted state, by flashing with a bad ROM or a failed flash. As Pi has no ROM, you can't brick it like that. Pi runs off the SD card only, so a clean reflash of the SD card should fix it.
If a particuar card is not working (and has never worked) there is a list of known problem cards on a wiki: http://elinux.org/RPi_VerifiedPeripherals#Problem_SD_Cards