First thing I'd do is turn the power to the furnace off for 30-seconds or so, then turn it back on and see if it clears the error.
Every one is a little different. A typical sequence is: call for heat, apply power to the igniter (if a coil), turn on gas, check for flame, if no flame, shut off gas, run a purge cycle, try again. It will often do this several times until it quits and says there is an error.
The manual should show the 'normal' operation.
If it has a sparker, you should hear the clicks when trying to light the gas. If it is a heating coil, you won't hear anything, but there will be a longer delay between call for heat and gas coming on. Sometimes the coil has a dual purpose: ignite the gas and then detect the flame. If it has a sparker, there's a separate flame sensor.
Any bad component can cause it to not light.
Could be a bad gas valve, or a plugged gas valve, or a shut off supply. Lots of things. Often the interlocks are fairly easy to diagnose, but you need to be able to read and understand the manual.
If this is a closed combustion system, a cracked intake pipe will also prevent it from coming on. This is often checked with a vacuum switch interlock.
If the controller has other lights, the 'normal' sequence would be described. You'd want to watch what's happening with them, and it might lead you to understand what function is not working.