It's hard to say anything specific from such a general problem description, but...
Is that drive is offline, then the RAID0 is offline and won't be mountable.
If you have critical files you might force it online and hope it stays online long enough to copy off the important files.
But generally what you say here is correct.
It's RAID0, so I assume if I can't repair the drive that I can add a new drive and restore from a backup?
Note, if these are the original drives that shipped with the Pegasus, there is a possibility that other drives might fail. Years ago a datacenter published a drive FAILURE study involving at a huge number of drives. The most interesting result to me was that when one drive failed, there was usually another drive ready to fail. And I've actually seen this happen.
So it might be an idea to consider buying new and larger drives and using RAID5 instead of RAID0.