Reinstall angstrom image

I just bought a BBone clasic and after I update and reboot it I couldn’t access again.
Now I’m trying to erase the SD card and reinstall the last image of angstrom but a problem with permission doesn’t allow me to erase or extract the image on the SD.
Is there any way to fix the SD? if not, how can I install angstrom again on the SD?

Patricio

http://circuitco.com/support/index.php?title=BeagleBone

Gerald