There was an error creating, configuring, or bringing online the Physical ... Creatingthe physical disk resource for 'Cluster Disk 2'  in failover cluster manager.

All the disks are offline.How to make them online in failover cluster manager??

Note: In computer management all the disks are online

So, you already build a cluster but the disks are offline in the cluster management -- 

you can right click on the disk in the storage  in the cluster management

and click on "bring this resource online."

When you ran the Failover Cluster validation Wizard, did it report any warnings on the storage? Because when all the disks are fine, they all get added to the cluster when you create the cluster.

Please check the cluster critical events and accordingly inform storage team to identify the issue.

disks are not coming online.

They are in failed status

Yes i got warnings in the validation wizard.

Disks also disappeared from windows 2012 server.

Can you please share the exact link for cluster critical events??

from failover cluster manager go to storage and see if you can see the disks.

choose and try to bring online. If they are not configured correctly it may fail.

Let your storage or network team know the issue. They will fix it.

warnings int eh validation report is ok as long as you do not have other errors.