Volume Won't Mount Help!!

I have a 4-bay Drobo purchased in 2011 that I have had no problems with until now. I have my drobo divded into 2 volumes - one for time machine and the other for photo files and documents.
As of yesterday, when I turn on my Mac, the time machine volume appears but not the volume with the rest of my files. Lights on Drobo remain green and dashboard says it’s in good health, but even the Dashboard doesn’t recognize that the volume other than time machine exists. What has happened and is there any way to fix it? I thought if a drive was corrupt the drobo would reconfigure to save files. What am I not understanding? Please help!

Try using Disk Utility/Warrior to check/repair the file system of the volume.