A Standby database can be down for many reasons.
Run these commands as the oracle user.
If the standby database is not running, start it using following command:
cd <DBVISIT_BASE>/standby ./dbv_oraStartStop <DDC>
Is it possible to provide more details around the issue, so we can respond accordingly.
Error messages and codes, excerpts from logfiles will be very helpful.
Thanking you in advance.