If you lose an Oracle archive redo log then the first thing to do is a FULL BACK UP YOUR DATABASE. Use rman to do this if you dont want to shut the database down.
With a lost archive log file, you wont be able to recover your database from any backup taken before that log sequence was created.
You should look at rman to manage your Oracle db backups. It will remember all the backups, look after your archive logs and allow you to create standby / clone servers in a zip.