It was working with no issues for last 3 years. Today it suddenly showing this error. Any help appreciated.
Bit of Nest history
Nest often has a sub base issue. Usually the issue is contained to the cooling side. Rh is power input for the heating side. So that wouldn't be my first guess.
Remove Rh and G and connect them together with a wire nut or simply twisting together. It doesn't have to be a very good connection, it just has to be a connection. This will bring your fan on. Now what you will want to do is, move to a location where you can hear the fan running. If it is running that confirms power. If it's not running you will need to identify the issue it may be a breaker, the furnace switch is off, the fuse in the furnace is blown, or one of many other power related issues.
If it's running
If the fan is running you can remove the RH wire from the G terminal and connected to the W wire. Now after a few minutes you should have heat blowing through your vents. If you do have heat blowing through your vents then I would suggest calling nest and getting them to replace your sub base they usually do it for free.
If you're unable to resolve the problem due to it being a power issue then that is another question which I am happy to help with but for now try the method outlined above and let us know how it worked.
I called a technician and he found out the cause is faulty Nest thermostat. I was using Nest 3rd gen thermostat. He removed the Nest and installed my old Honeywell non-wifi thermostat and my air conditioning started working again. It's sad that $250 Nest stopped working in 3 years. Mostly I won't go back to Nest again, I'll use Honeywell.