rman cold and hot backup

I ran an RMAN backup.  It looks like I lost one of my archive log files. 

RMAN-03002: failure of backup plus archivelog command at 10/25/2012 11:01:15
RMAN-06059: expected archived log not found, loss of archived log compromises recoverability
ORA-19625: error identifying file /u02/archive/RMANT/arch/1_1_797519559.dbf
ORA-27037: unable to obtain file status
IBM AIX RISC System/6000 Error: 2: A file or directory in the path name does not exist.
Additional information: 3

Since this is test,  I decided to stop the database, mount it and do a   alter database noarchivelog;

I then stopped the database, started it mount and ran a "cold" backup.  This was successfull.   I then stop database again, startup mount, and did alter database archivelog;