Sorry, forgot to add that the diagnostics show the drives as being ok. Any guesses as to how to get it to boot?

Check to see if one of the drives is showing a failure (or run the Dell diagnostics - F12 at boot). It sounds like one of the drives is gone - if so, there's not much hope unless you go with data recovery. If you decide to do that, do nothing else to the RAID 0 array (if that contains the failed drive) - you'll need to send both drives (and budget about $5,000 for recovery).

Which diagnostics did you run? The quick 10-minute test or the extended test? If you ran the quick test, re-run the extended test.