1) Holding "CMD + R" and boot your Mac (you can release the key once the loading bar appear)
2) Once boot into the recovery partition, choose disk utility at there
3) Click your HDD on the list (the disk, not the partition)
4) Click the button that above the big HDD icon (the left most button on the top)
5) If nil help, click the OSX partition, and try the first aid button again.
6) If no fault found, click the HDD and then click 2nd left button, there should be 2 partitions at there (the white one is the space you want to recover)
7) try to adjust the blue partition to recover all available space (you may need to delete the white one, make sure didn't delete the OSX partition).
8) Triple check you make the correct adjustment, and then click "Apply" (bottom right )