Do you mean a record has been deleted or the serial number field has been accidentally cleared. If the latter, the serial number field should never be accessible to anyone ever. If it's gone it's gone. Do you have a backup? if so you could take a look and find the missing number.
What Rick said.....but if you need to know what that number was, If you have the primary key from the primary record on a child record layout, and you know a particular child record, you could find it there. I usually put the pk on the child record offscreen.
Also, if the Records were entered in order, and never deleted, you could look at the records in Table View, and try to find a missing number.
Easiest, is to go look at a back up copy (I'm assuming you can't do this because you wouldn't have asked).
Backups! Backups!.....the more the merrier. I'm sure everyone has been burned by not having a back up (hopefully only once).
Santosh, if you are trying to know what records have been deleted, here is a fairly simple script which I created (long ago). It requires one thing which is not visible in the script, which is a "self-relationship", from your table to another table occurrence (of itself, same table), on the Relationship Graph. It would be from a global field to the ID field. [I called it "code" in the script, as it could be from something other than an ID.]
I just adds 1 to the "global code", the tests whether the relationship has a match. If not, it keeps that code in a list (global). At the end it Exports the list of "not there" to a text field; it is also in the global, but that would not keep if you're on a server.
I don't know how many records you have, etc., so there may be a faster method, if this is too slow.
I want only to short out the missing ID no. list