You may see this message in RAC when trying to start up instance or database. This error is being caused by unmounted ASM diskgroup.
ORA-01157: cannot identify/lock data file 37 – see DBWR trace file
ORA-01110: data file 37: ‘+MIDATA2GRP/midb/datafile/iotest.256.772203865’
You can solve this by mounting the disk group.
login to ASM instance as SYSDBA
sqlplus / as sysdba
SQL> Alter diskgroup MIDATA2GRP
SQL> exit ;
Now try starting up the instance
srvctl start instance -i midb<#> -d midb