Assuming it’s a local file and there are no replicas/copies (either locally or on Domino servers), you may try to fix the db running either fixup or compact on it.
(Make a copy of the affected db before you attempt next steps!)
Client execs for both methods (there are also server versions) are located in Notes directory of Lotus install and called nfixup and ncompact respectively.
Run nfixup -? or ncompact -? to see syntax
Warning: if no database is specified it runs on all databases!
IBM suggests to run fixup first (you may try various options) and if this fails to try compact -c to salvage anything which is still readable (this runs a copy type compact). You may also use addtional options (eg. -D to discard view indexes or -i to ignore errors).
You may also need to run updall to update/rebuild view indexes after recovery.
Or try: NSF Repair Kit. A must-have Lotus Notes NSF repair utility for home users and professionals dealing with NSF file corruption on a regular basis.