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Due to corruption of NSF file all your Lotus Notes Database becomes inaccessible. In such situation you can restore the Lotus Notes from a backup but, if you do not have any backup then, you need a Lotus Notes Repair Toolbox. This tool efficiently scans the corrupt NSF file to recover your vital data with the use of advanced and indigenous algorithms.
Here's a good primer on how to recover corrupted Lotus Notes files.
Basically, though, if you need to recover corrupted files -- and assuming you don't have a backup -- you need an NSF file recovery tool.
I've also read that if you "rename the file then force a replication from a known good server," it will rebuild the indexes -- provided the file isn't too corrupted.