why won't my engine turn over?

TEST AND VERIFY battery voltage/condition. Then test and verify proper switched voltage to starter solenoid when someone tries to crank it. Depending on those results, either replace starter or other failed portion of circuit (assuming ignition switch itself is good) . Make sure engine will turn by hand (wrench on crankshaft or other means as needed).remove and inspect plugs/cylinders for signs of other internal problems.