problem with induction sensor

I seem to have a reliability issue with my induction sensor.

Basically.......printer was working fine up untill about 2 weeks ago, then I left it for ages as I was busy, tried to run a print this weekend and when homing the z-axis the bed keeps going through the printer head, even though the induction sensor is plugged in and the light appears when the metal bed gets to close. Anyone know the type of issue I have? extremely frustrating!

Thanks in advance

Check the connection between Sensor and Signal on ramps.
If you have a multimeter check Volt between sensor signal out and ground in both state (light of & light on) to see if something change.
If not, the sensor is out OR the resistor OR the 7805 are out.