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one thing that you should be aware of is the lump needs to be well lit before throttling back by adjusting your vents for a low and slow cook, otherwise you risk stifling your fire and eventually extinguishing it.. the end result is probably what happened to you assuming that you had enough lump to begin your cook.
one method that i have recommended to the new egger is to get yourself a chimney starter(about $12 and can be found anyplace you can purchase barbecue equipment). load it with lump and follow the directions attached to the chimney starter to light the lump.
now fill the egg with lump to your desired level. for long low and slow cooks you may want to fill the egg to the top of the firebox. once the lump in the chimney starter is fully lit, carefully pour the well lit lump into the firebox of the egg distributing the lit lump across the top of the lump already waiting in the firebox. this ensures an evenly lit manageable fire.
close the dome of the egg and set the top vent approximately 1/8 to 1/4 of an inch open and the bottom vent open approximately the width of a credit card. Now wait!!! the egg's temp may initially indicate temps well above your desired temp, however after a few minutes and as the egg's thermal mass begins to absorb the heat of the lump the temps will settle to a dome temp at or near 250.
although you may discover far easier methods to light the egg, this is one method that will ensure good results without "losing" the fire over night.
Of course, you may want to check out the naked whiz's ceramic charcoal cooking webpage for additional tips on temp control.