Our Bishop closed our parish. My grandparents were among those who built and paid for this parish. By what right does the bishop close what is ours?

Canonically, there likely are some solutions that permit the lay faithful to take possession of a building slated for closure, undertake its maintenance and keep them open as chapels etc. under the supervision of the local church. Frankly, though, most congregations that have reached a critical state where closure is deemed necessary are not in fact able to undertake such solutions. 
 
While there are legitimate canonical issues, as the lay faithful you have canonical rights at the closing of the parishes, I am not a canon lawyer and would like to answer your question pastorally. 
 
And from a pastoral point of view, it seems evident that bishops do not close parishes, people close parishes. Some wish to explain the widespread closing of Catholic parishes, especially in the Northeast, as mere demographic shifts. And while there are demographic issues, the fact remains that with the Catholic population almost double what it was in the 1950s, many parishes filled to overflowing back in that era now sit increasingly empty. 
 
This is a teachable moment, and we must accept some very painful facts. When only 25% of Catholics go to Mass nationwide, and when Catholics stop having many children, or effectively handing on the faith to their children, this is what happens. 
 
The Church simply cannot maintain parishes and other institutions such as schools and hospitals when Catholics are largely absent. Pastorally speaking, people, not Bishops alone, close parishes. Many parishes, schools, seminaries and convents now sit largely empty. And as they begin to go empty, bills are unpaid, maintenance is deferred, and the situation eventually becomes critical. Dioceses do not have endless amounts of money, or priests and other personnel to staff and maintain increasingly empty, no longer viable parishes… Decisions have to be made. 
 
Pastorally, one would hope that long before things go utterly critical, that bishops, working together with communities that are going into crisis, can speak honestly and work for solutions. But this is not simply the responsibility of the Bishop, it is the responsibility of all the people of God to have such honest discussions. 
 
Thus, we are left with a difficult but teachable moment about what happens when the faith handed down to us is largely set aside by the vast majority of Catholics. It's time to Evangelize and make disciples, as Christ commands.