CMOS error 251 shows that a memory module has failed. Your lightning strike may have done more damage than you think. Try reseating your memory modules and reboot. If it fails again, remove DIMM 2 and reboot. If that fails remove DIMM 1, replace DIMM 2 and reboot. Try them individually in other DIMM sockets too. If you can, try them in another mobo (a friend?).
If no luck then you may need to buy new RAM - or even a new mobo too! First on your shopping list should be a surge protector strip for your PC and peripherals.