very big transaction log file

1. Have run transaction log backup every 15 minutes. It is fine for a while. All of sudden, transaction log file grows big in short period and cause transaction logbackup to fail(disk space issue). What could be the cause and how to fix it?

2. One existing transaction log file of one database(Full recovery model) is more than 300 GB. Any way to reduce its size without losing anything...?It is production box...

Thanks