Whenever you turn archiving on for a database, you must take a backup right afterwards, before any transactions have been done. You cannot use a backup taken while the database was in noarchivelog mode to restore a database that has been put in archivelog mode.

Consider the following scenario:
1) database in noarchivelog mode
2) cold backup
3) turn archiving on
4) cold backup
5) open up to users
6) hot backup
7) database crashes

Database can be restored using archived logs and hot backup from step 6 or cold backup from step 4.
Backup taken at step 2 cannot restore database to the state it was right before it crashed.
It will only restore to the state it was at step 2. Archived logs cannot be applied to a cold backup taken when the database was in noarchivelog mode.

hope this clears it up