1992 mercury topaz wouldn't start the fuel filter was changed and now when running the oil light comes on and sounds like the rods are knocking then will not start again could this be the egr valve any simple solutions?

It sounds to me like you've run out of oil completely. You may want to check that. A low oil light could mean you're low on oil. AKA you have less than 1qt left and that doesn't bode well with most engines. I wish you the best of luck with it though