If you have a block heater then plug that in. Else, assuming its not a cranking issue, then let the glow plugs warm up more by waiting another 10 or so seconds after the WTS (wait to start) light has gone off before cranking. Some ppl also cycle the glow plugss by turning em on for 30 seconds, off for a few seconds, on for another 30 and then finally crank. If the glow plugs still won't warm up, check the glow plug relay (GPR), located on top of the engine beside the fuel bowl. The GPR should click when the circuit closes (its a solenoid) - if not, jump its two big connectors with a screwdriver (or something) to bypass the GPR but only do this for like 30 seconds.