you can find the archive log gap using v$archive_gap. gap between low sequence and high sequence. this gap will happen because of space issue,network issue etc.. if one archive log file is not applied from prod..that file is deleted in prod itself..remaining archive logs available in standby ... it won't apply in standby db... that gap u can find it using that view.. for that u can reconfigure again otherwise find scn from standby. based on that scn take incremental backup from prod.. and controlfile..and apply on standby and start with new controlfile...standby will sync without prod downtime..
1) archive log corruption
2) unable to transfer as size is big and network speed is low
3) any configuration of dataguard issue
4) archives are deleted from OS level, or forcefully deleted?
5) or the standby is behind from many days.
If gap is more then perform incremental rollforward , check this link http://www.oracle-ckpt.com/rman-incremental-backups-to-roll-forward-a-physical-standby-database-2/