If it won't even recognise the disc then as cholla says there's very little you can do.
Is there a chance that the disc has been physically damaged or even just needs cleaned?
Hold the disc up to a light source and reflect it in the disc. When you hold it at the correct angle it's very much easier to see any scratches etc.
If it's specks of dirt or fingerprints it's very easily cleaned off with either soap and water, or isopropyl alcohol, or methylated spirits on a lint free cloth.
Methylated spirits and isopropyl alcohol are weak solvents that evaporate quickly so don't use any solvents as you could damage the polycarbonate of the disc.
Scratches can be repaired in different ways and you can either try CD repair fluid, which fills the scratches with polymer, or try various methods of polishing them out.
Video stores quite often have machines that can do this for you although video stores are a dying breed these days.