Unable to Mount ASM DiskGroup

if you try to mount it you will get an error some thing like the below

SQL> alter diskgroup DATA1 mount;
     alter diskgroup DATA1 mount
     *
     ERROR at line 1:
     ORA-15032: not all alterations performed

to do so issue the below command

oracleasm scandisks

and then bring the diskgrop online.

SQL> alter diskgroup DATA1 mount;

           Diskgroup altered

Done..!!