Hi,If the starter is not operating when you turn the key any longer, the first thing you want to do is have the battery checked to make sure it is not depleted and that the connections are clean and tight.
If the battery tests good and all connections are clean/tight then you would be most expecting either a faulty starter or (long shot) an internal bind in the engine preventing the starter from rotating it. From that point you would want to take a 1/2" ratchet and socket (usually 22mm on these) and try to rotate the crank pulley clockwise (Through the wheel well). If you can not rotate the engine clockwise and hit a dead stop, then there is an internal failure in the engine preventing it from rotating.
If the engine spins without too much effort (some compression resistance is normal, just no dead stops), the battery is testing good and all connections are clean/tight yet when you turn the key the starter won't spin the engine over, it will be a faulty starter.