I will say if the base is still firm, then you just try to grow it again but watch closely
gradually increase the light exposure.
If you give it enough space and light to grow, it will grow back to a nice round shape. I used to have one cactus ( not sure if it is the same species) that was also elongated but not as bad as yours. After sometime it grows 3 times in volume, into a nice tennis ball shape, you still can see in the picture that the base is very small,
gradually increase the light exposure. 
