One drive missing usually means it has failed or has been disconnected. Since this is raid0 (striped set) that would mean the entire volume is useless which is why it's offline. Formatting is not going to fix this.
Disk Utility RAID user interface is really screwy, so confusion is normal. But in recent versions you can click on physical drives (not volumes or raid sets as you have done here) and click the Info button in the tool bar, and find the SMART status which should give you an idea if the drive has in fact failed or not.