urgent help - database are not mounted

Hi all,

We have one dag with three servers accross two AD site.

Site1 : 2MBX serves

Site2: 1MBX server

Now, the MBX servers are down due to outage.  But, the dababase are not mounted on the server in site2.

I can see that DAG in the failover manager is down.  How do I bring up the DAG? also, the file share witness failed and how do I change the file share witness?

Please help.