speedo?

Hi, just changed the head light on my 02 mille, now speedo does not read, all the rest of the display works fine any ideas? I known the sensor on the rear wheel can be a little tempremental, may be a this just a coincidence and this has broken , How do you test the sensor? have aready givin...

Just did the speed sensor mod but speedo still not registering a speed. The red led on the end of the new sensor flashes every time a bolt head passes. What have I done wrong or are there any other common speedo problems?