what Oracle version are you using is even more important than the OS. And is this is cluster or a standalone ASM instance?
If ASM does not discover all discs from a diskgroup it cannot mount it. What could have been the issue, is that ASM tried to start, before all disks were online.
Which would clarify why after restarting cssd everything worked.
Normall ASM are stopped and started (if not 11gR2) with sqlplus. However if a diskgroup does not mount, you normally only use an mount command:
export ORACLE_HOME=<your Oracle Home>
sqlplus / as sysdba
SQL> alter diskgroup <DGNAME> mount;
To start and stop:
SQL> shutdown immediate;