Our 2 node 10g RAC database would not startup after being shut down. The database was generating an error after trying to startup with command
srvctl start database -d db1
It turns out that one of the other DBAs modified some database parameters sga_target to 1710 and forgot to add the M or convert it to bytes...ouch!
Somehow one of the nodes got unregistered from the cluster.
The following command was used to add back the instance to the cluster
srvctl add instance -d dbname -i instance_name -n nodename