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.