why wont my oil come out of the pipe to my burner?

hi, i have a reillo 40 burner and i ran out of oil the other day, since then i have taken apart and cleaned all the parts and filters from tank to burner and still wont light.
there is no oil coming out of the pipe going into the burner and i tried sucking only to get a small trickle then it stoped, i tried running it a few times to see if it would draw it down the pipe but still wont come out.
my oil tank is higher than the burner.
please help!.
regards, p.mayne.