I'm quitting Minoxidil on account of no change after 6 months. If I stop now, will it make my hair fall out suddenly?

It typically takes 6 months to see results of minoxidil, so I would suggest waiting a full year before considering stopping the medicine. Alternatively, if minoxidil does not help in the first place then there should be no negative repercussion from stopping use of minoxidil. If minoxidil was working to prevent hair loss, then that hair will shed, and you will revert back to your original pre-minoxidil state.

We do not know how was your hair before minoxidil, if it worked, quitting minoxidil won't give you a sudden hair lost, but you can observe it with time. İf it helped you with the hair growth keep using it for 1 year. 

Yes, I would highly recommend not stopping Minoxodil. If you were only in the first week of using it you would be able to stop. However, since you have been using it for 6 months if you stop now you are at risk for shedding and shock loss. 

Typically when you stop minoxidil you only lose the new growth you've gained and go back to your baseline.  Note that it can take 9 months or longer to see a difference.

If you have had no effect from minoxidil, then you are under no risk from the withdrawal of the medication