Yes, as long as you have a backup of the metadata. "Metadata" stores the name, RAM size, and attached disks for each VM. Without the metadata, all you have is a collection of VHD disk images with no names, and virtual machines!!!! If you lose your metadata, you don't lose any of your data, but you must manually piece together each vm, and which drives went with it. What a pain!!
Here's how to configure backups so this doesn't happen to you:
You will need to decide where to store your metadata backups. If you only have one iSCSI drive, then use that. From XenCenter, right click on the Storage Repository where you want to store your backups. Choose "Set as default". This will put a black check mark over your SR. If the black check mark is already there, then your SR is already the default.
Click on the host machine in XenCenter, and click the console tab (or ssh into the XenServer host).
Type "xsconsole" into the terminal. Choose "Backup, Restore and Update". Choose "Schedule Virtual Machine Metadata". (enter your root password if it asks). Choose "Daily". Choose "OK"
Now you have scheduled backups every night! If you want to make a backup right now, choose "Backup Virtual Machine Metadata".
In the case of a total failure, you will need to install XenServer, attach your iSCSI, and then do a "restore" from this same menu. After the restore completes all your VM's will showup in XenCenter!