How can I figure out why my light won't turn on?
It doesn't sound like you can yet definitively conclude that the neutral line is bad, however an easy way to verify a bad neutral connections would be to test continuity between neutral and ground at the light.  You can do this even using a cheap voltimeter by setting it to a low resistance setting.  More advanced voltimeters may have a special continuity check that will beep when it detects 0 resistance.



Since you have neutral and ground bars connected at your central box, you should show near 0 resistance between neutral and ground at the light.  If it is 1 or a much higher number than you have infinite resistance meaning that your light is either not grounded properly, you have a disconnect somewhere on the ground wire, or you have a disconnect somewhere between the neutral wire and the neutral bar in your central box.

NOTE: You describe a two way switch, however this diagram shows what is called a three way switch in the United States.  The load from the central box or from another receptacle on the circuit is either entering at one of the two switches or to the light itself.  Identify which of these wires carry power and inspect the wire connections at the light for ground, neutral and hot at the switch boxes and at the light.  Neutral's should be connected directly and not switched.

If these connections seem good then perform more continuity tests between ground and neutral at all locations to help identify if a wire is bad.  More than likely the disconnect is a loose crimp or wire nut at one of these three locations.

If you still cannot find anything wrong at all three locations then the problem is likely at a receptacle or luminaire further down the circuit, as you mentioned that you are also experiencing problems with lights ahead of the circuit as well.

CAUTION: Many European single phase systems operate at a high voltage.  As always be extremely careful and take your time when handling electricity!