Right lets go through the check list.
If you could please replace both cards in the slots you were using, Slot #1 and slot #2
Ensure the PCIe toggles next to the Power and reset button are enabled.
Ensure that your power connectors to the graphic cards are firmly seated.
Clean the SLi fingers on each card. Make sure there is no dirt or any oils on the contacts.
Push the SLi bridge onto the SLi fingers. Firmly holding the card at the back make sure the bridge is fully pressed on.
Power up go into BIOS.
F5 - enter
F10 enter
Pleaser report back what the result is.
To help us help you - please provide as much information about your system and the problem as possible.