If a single drive drops from a pool then it should have gone into read-only mode and duplicated files should still be present in the pool, regardless of why the single drive dropped out. If the drive is still present in Explorer / Disk Management and seems to be okay, but is no longer in the pool, DrivePool's metadata (the bit that says "the hidden poolpart folders on drives A, B, C, etc are part of pool X") may have been damaged.
I'd try a Manage Pool -> Re-measure... and if that doesn't help try Cog -> Troubleshooting -> Recheck duplication... and if that doesn't help I'd consider lodging a support ticket with Stablebit since the metadata is supposed to be stored in a triply redundant fashion where possible to prevent this sort of problem.