[I had purchased a couple of PR1 and PR2 domains. Now, I have lost its Pagerank. Can you please tell me how to regain PR? Should I build backlinks to them?]
Domains don’t have PR. PR, or PageRank, is assigned to individual web documents (URLs). If you do not keep the URLs intact, that inbound links were pointing at, then you will lose that PR. If the web pages that were linking to your URLs have your page’s link removed then your page you will lose that PR.
Often when someone purchases a domain from a preexisting website they will change the URL structure which causes the existing inbound links to be broken and all the PR derived from those links are lost. If you can, restore the previous link structure and you may see the PR return.
You don’t re-structure a domain, you architect URL structure. If you have backlinks pointing to a particular URL, you should either recreate the same URL or redirect the inbound requests to an existing URL.