I'm using zenoxidil/minoxidil gel 10 for past 20 days good results have been observed but after stop using it what will happen ?

Hair loss is a life long process.  If you are responding to treatment this is great as a small percentage respond to topical therapy.  The hairs that have grown with the use of minoxidil will be shed once you discontinue its use.  

If you see good results with minoxidil but wan to stop using it, the results you gained will not last. You have to use the product indefinitely.

Minoxidil use for men and women with androgenetic hair loss (also called male and female thinning) is meant to be lifelong. If one stops, any benefits that were experienced will disappear and hair loss patterns will return to the way they were before treatment.