Currently, I have setup log shipping from production server to DR server. Now, the doubt is that whenever my production server goes down or lets say data center crashes, I will switch it to DR site. On DR, I will change the mode of database from "Restoring/No Recovery" to "Recovery" mode. But, now the problem is that whenever my data center recovers lets say it recovers in 2 hours, then I want to switch again to my production server. So, when it gets switched to PROD server then how can I change database mode to "Restoring" state on DR site to able to start log shipping again?
This article covers the topic: https://msdn.microsoft.com/en-us/library/ms178117.aspx
Basically, you have to configure log shipping from the former secondary (now primary) server to the former primary (now secondary) server.