It can be really difficult to find a comfortable position to sleep when you have back pain. There are several possibilities you might try; the only way to know which works best is by experimentation.
You can sleep on your back if you have some pillows under your legs. How good this is for you probably depends on exactly which disks are having problems though. I personally find this most comfortable when I have two pillows placed just under the knees. If you don't have pillows then this position puts a lot of strain on your lower back, which can be partially reduced by spreading your legs, or bending one knee and sticking your foot under the other knee, kind of like a pillow.
You might also try sleeping on your back, but using a lumbar role just above the belt-line. Sometimes this feels good to me, but usually only when I'm in a lot of pain already. It can be tricky to keep the roll in place. One option is to use a rolled up towel, draped across the bed, or perhaps even tied around your waist (I've never gotten that to work, myself) .
Sleeping on your stomach is also pretty good; I find it's much harder on my neck, but my back likes it. It's good for short naps, especially since I don't find it hurts my neck too much then, and really relaxes my back.
In general, however, the position which I find comfortable most often is sleeping on my side, with a relatively thin pillow tucked between my knees. I almost never wake up in pain after sleeping in this position.
I encourage everybody to experiment with all the different sleeping postures. Also keep in mind that what feels good today may change tomorrow - I've switched between sleeping on my side and other postures several times, usually depending on how bad the pain is. Don't get too set in your ways, and be willing to try changing things around once in a while.