Did you backup your SD partition image? If yes, restore it. If not, you should use partimage (available in ubuntu) to backup your SD partition regularly. You can then easily restore the whole partition under this kind of bad scenario.

For now, you can try to check and fix your SD card inside a unix machine (say ubuntu -- partition editor). The file systems might be corrupted. You can then try again booting from the SD.

In the worst case, you got to reflash the unit.