Any information you can see from alert log?
Have you used any of the clauses NOWAIT (or) IMMEDIATE with the commands? They can faster in terminating recovery process.
SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL [IMMEDIATE] [NOWAIT]
Please run step by step
- STARTUP NOMOUNT
- ALTER DATABASE MOUNT STANDBY DATABASE;
- ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE DISCONNECT FROM SESSION;
Hope the below note would be helpful in trouble shooting your issue.
How to resolve MRP stuck issues on a physical standby database? (Doc ID 1221163.1)