In Explorer, after performing factory reset Google play store converted to Market place I performed factory data reset of my another device "HTC Explorer". And before performing the reset Play store was there but now it disappears, and instead of play store now its Market place. So how to convert Market place to Google Play Store again?
Google Play was formerly Android Market. Your phone would originally have had Android Market (I am assuming its an old one, probably Gingerbread). When you did a factory reset all the updates were reverted back included on Android Market (aka Google Play).

Android Market automatically updates itself so give it some time and it should update itself to Google Play.