OK, so the climate control blower isn't kicking on. There can be a lot of reasons on these automatic setups.
It could be a failed blower motor (due to worn brushes, usually).
Or a further failure of your push button climate control panel.
Or a failed temperature regulator.
Or a failed blower relay and/or resistor.
One easy way to begin is to verify whether your blower motor is good. To do this, simply take off the kick panel on the passenger side, unplug the wires, and remove the motor by taking out the 3 or 4 screws holding it in place. You can then run a 12v lead to the motor from your battery and see if it spins. If it's spins easily you can be fairly sure the problem rests elsewhere.
If your fan is good you'll need to diagnose the problem with your system. One fairly easy way to do this is to get your hands on a full spare set of parts (climate push button panel, temp regulator, blower relay, resistor) from a parts car. Then you can do some parts swapping to see if and how the system performance changes. I got lucky and this worked for me - I swapped in a spare temp regulator when my blower didn't work and viola, blower came back and works perfectly through all the speeds again.
If you don't have spare parts on hand, or want to do the more thorough job of testing the various components with a multimeter, see this guide.
http://www.benzworld.org/forums/attachments/w124-e-ce-d-td-class/512915d1367573783-middle-aged-mercedes-electrical-repairs-middle-aged-mercedes-electrical-repairs.pdf
The section on diagnosing the W123 climate control starts about halfway down on page 6.