When I turn the ignition key absolutely nothing happens, no noise, clicking. I replaced the battery and still the same

Most likely, the problem you are dealing with is a dead battery. The reason this is apparent is you said "there is no clicking noise like a bad starter, it just does nothing..." Normally, if there is any charge at all in the battery, you will get the clicking noise you describe as the ignition circuit is trying to charge the solenoid that turns over the starter and turns on the engine. If you have a battery jumper box available, your S40 will probably start at the first flick of you ignition key if your battery is fairly new. To hook up the battery box, just place the clips as you would standard jumper cables, red to red (+) on the battery and black (-) to a good ground point on the engine (it has to be solid metal-to-metal contact). Then turn key to fire up the engine. Once your car has started you have to run it about 10 miles to put any sort of charge back into the battery. If the battery won't hold the charge then it is time to replace it.

Is it a manual or automatic? When you say "won't turn over" does that mean that absolutely nothing happens when you turn the key? Or does it only click? Or does it crank but won't fire? If it is a manual and nothing happens when turning the key, it could be the clutch safety switch is faulty.

I looked at info for your security system, it is starter motor disable. I didn't see 06 ford Taurus in my database, so I looked at 05.
Battery has a full charge and battery connections all good?
We can usually do some tests to see if starter motor will crank the engine, but, if the security is the problem, it still won't start.
You can check for applicable trouble codes? Try a different key?

On the fuse box lid should be diagram of relay positions , The relay that's clicking ,what's it for ?

You need answer some questions first to rule some things out.
Is the starter making a noise when you turn the key or is it just clicking? If the lights light, but the starter makes absolutely no noise, then the power cable to the starter is bad or contacts are corroded. If you hear a click every time you turn the key but that is all, then the starter is bad and needs replace. Once it is removed, most parts stores can test it for you. If you can hear the starter spinning but its going real fast then again the starter is bad. If the starter is working properly (like on a really cold day when it doesn't start right away) but the car still won't start, then the starter is fine and your problem is either in the ignition or the fuel system. You may be able to narrow it down if you can read the codes, otherwise it needs a mechanic and you need to decide what is the car worth to you.

I just went through this two days ago. You probably need a new ignition. What happens is these cars lose connection for some reason to the chip in the key and it can't be fixed with just programming. It has to be a whole new ignition. Mercedes are so much different then to any American car. Good luck and it costed around 950 to fix in Wa state.

Go out to the battery, loosen it up, run some sandpaper to get the terminals nice and shiny, on both the battery and the wire connectors, and then tighten them up very well. If that doesn't work, pull the battery out again and get it charged up.

Thanks for choosing FixYa and welcome.

If absolutely nothing happens (no noise) then the problem is most likely either in the starter (my first guess) but it could also be a faulty ignition switch. Sometimes, if you can even get to it, you can tap on the starter case with a hammer and get it to work again, but even with this it would be time to invest in a new starter.

It could be the park/neutral switch, next time it doesn't start try wiggling the shifter in park while turning the key, or shift to neutral and see if that will work. if that doesn't change the problem I would replace the ignition switch.