macOS – RAID 0 Setup Not Showing Full Disk Space

This sounds like your hard drive failed entirely. I see that it's a typical SATA drive in that model. You should be able to open it up and remove the drive, then wire the drive to a desktop and see if it is detected. If it is not, that confirms that the hardware failed.

Comment by OP: It turned out to be a SATA cable failure, see Hard Drive Issues with MacBook Pro 13-inch mid-2012.

The first thing I would try is to open Boot Camp Assistant and see if you can remove the partition. Whether Windows was installed or not, this should be possible.

Alternatively, since I'm guessing you haven't repartitioned your drive since you installed El Capitan, you won't be able to resize the partition unless you use a source outside of your computer. Even in recovery mode, you won't be able to repartition. You might be able to resize it in Disk Utility if you use Internet Recovery: CMD + Option + R at boot. Another option would be to boot into recovery mode, install OS X on a USB drive, then from the installation on that USB drive, you would for sure be able to resize your partition.

I have done this many times, so I understand the frustration it can cause when you have space that you can't recover and the only way to get it back involves several hours of your time.

The good news is that if you ever start with a fresh partition in El Capitan, their new partition scheme is resizable even when you're using it live so you wouldn't need to ever even touch Recovery Mode.