Try these trouble-shooting tips:
Make sure that the transformer is plugged into an electrical outlet that is working. If there are any electrical switches that control the outlet, make sure they are in the "on" position.
Make sure the transformer is the correct size and can accommodate the total wattage of your system.
Check that the timer settings on the transformer are correct. To check the system during the day, cover the photo-eye with a piece of black electrical tape.
Check to see if there is a loose connection at the transformer pressure plates.
Check the locations where the lighting fixtures connect to the low voltage cable to make sure that the prongs have pierced the cable.
Trace the low voltage cable from the transformer to the lighting fixtures to ensure the cable has not been cut.