It amazes me how often I see reports here of people doing this "by mistake". If you don't understand what a button does, why would you press it? And why would you then ignore the warning message (about losing contents of the drives) and say it is ok to proceed? I guess Intel needs to beef up these warning messages significantly.
Your hard drive is not offline, it is encapsulated in a RAID array and, as a result, is not displayed as a separate device. You need to remove the drives from the array. The problem is I am not sure that the contents of both of these drives will survive this operation. I do hope that you have backups...
Sorry, I wish I had better news,
Just what I said. You need to remove these drives from the array (saying it another way: make them non-RAID drives). I have found that it is easiest to do this in the pre-boot environment. If you have a modern computer with UEFI-based BIOS, then the interface to do this will be within the BIOS Setup program. For older machines, this is done by repeatedly pressing CTRL-I while the BIOS displays its splash screen. Once the GUI appears, choose the option to make the drives non-RAID and select the drives in the array (both your SSD and HDD).
Hope this helps. Sorry I cannot be more complete or specific, but how this works can differ from one motherboard to another (and one motherboard manufacturer to another).