What is an open ground? Also, why would there be 50 V in the neutral and ground wires? I have replaced all outlets and switches and still I have no light.
When you see a phantom voltage like that, it's the hallmark of a sensitive meter. Though sensitivity sounds like a feature, most cheap meters have it too. Disregard.
Well, we can read a few things from the tea leaves.
You know this wire runs near a "hot" wire for a little bit of distance - enough to pick up an SLF radio signal from the adjacent wire. If you've ever built/used a crystal radio that uses a tiny earpiece and is passively powered (from the AM airwaves themselves), you understand tiny amounts of power can be induced from radio.
You know this wire is rattling i.e. not connected to any supply. You know in a North American panelboard you have 4 wires: L1 that's 120.4 volts to ground, L2 that's 119.6V to ground, neutral that's 0.4 volts to ground, and ground that's 0.0V to ground. (assuming a 0.4V neutral-ground drift; YMMV.) You know two things from the rattling/floating wire.
- it is not connected to any of those (because if it was, it would read that voltage).
- it is not connected to a load whose other wire is connected to any of those (because if it was, the load would "pull the wire" to the voltage of the other wire.)
Measuring a voltage neutral to ground is referred to as a phantom voltage. When wires are run parallel a conductor with power will induce voltage on a non powered line. How are you measuring the open ground? Since you have a voltage there I would not expect it to be a bootleg ground that is not code compliant but found many times when folks that don't know the right way tie the neutral and ground together. It is possible there is nothing connected to the ground lug and the phantom voltage is being read because of a high impedance meter (most volt meters are high impedance) . replacing all the switches and outlets did you replace them exactly as they were? Some switches use a switch leg where the power comes in from a light fixture, most of the times in this case wire colors mean nothing. Is it an old 2 wire system that you added GFCI protection so you could have 3 wire outlets? GFCI outlets have a line side from the breaker and a load side to other devices if wired wrong it won't work. Last turn the breaker all the way off and back on this resets the breaker, if it immediately trips there is a short someplace, if it holds start checking for power at the outlet or switch closest to the panel as a starting place then work from there to find the problem. Additional info would be helpful but an open ground won't stop lights and outlets from working.