WAL Replication Working but Not Working

We're having an issue with our warm standby server. About 9:30 last night, it stopped applying changes it received in
WALfiles that are shipped over to it as they are created. It is still reading WAL files as they delivered, as the
startup_log.txtshows, but the changes in the primary database aren't actually being made to the standby, and haven't
beensince last night. 

Is there any way we can figure out what is going on here? We'd like to recover somehow without having to restore from a
basebackup, and we'd like to figure out what is happening so we can prevent it in the future. 

Thanks!