The most common reason for not heating up is because of a blown thermal fuse. Apparently the controller lets the iron get too hot or the fuse is out of manufacturing tolerance - and so the fuse opens.
The green lamp is normally lit when the relay is off and dark when the relay is on. When cold, the relay normally turns on within a second of power-up (switched on). During power-up, the green (ready) lamp should flicker on, and then as the relay clicks, go dark. If the green (ready) lamp does not flicker, the fuse is probably blown. A replacement fuse (240°C) can be purchased from many sources (Google search).
The fuse is clamped inside the bottom clamshell in silicon tubing. Do not damage this tubing. Remove the clamp over the silicon tubing. Slide the tubing to the side and cut the stranded wire close to the fuse. Remove the tubing and cut the other stranded wire close to the fuse. Be careful not to nick the wire when you strip off insulation. The fuse should be re-attached to the wires with new all-metal crimp fasteners. Don't forget to reinstall the silicon tubing before you crimp! You can get the crimp tool at hardware stores, Radio Shack, Lowes, etc. If the "iron" is getting too hot and blowing thermal fuses, you may be able to avoid this failure by keeping the thermostat set lower.

Just fixed mine with info found on hear. If you're a little handy or have someone who is it's a simple fix. Under bottom plate a thermal fuse is connected in black wire under white insulation. I picked one up at radio shack ( got the 228 dgree C.) I cut insulation and removed. I then cut black wire at connection at each side. I then placed new fuse in line and crimped small connectors on each side. Placed insulation back on and taped up. Screwed back together and it worked fine. Fuse cost $1.69. To think I almost tossed it.