It is important to be clear here about whether you mean there is no sound at all when you turn the key to start; which is really what happens when the starter does not turn the engine over; or whether the starter turns the engine over but it does not fire and begin running. About half the people who write in here misuse the term "does not turn over" to mean the engine turns over but does not fire. And the two meanings of that phrase point to entirely different problems.
If the starter does not make any sound when you turn the key; turn the headlights on, and check their brightness. If the headlights are very dim or don't light up; the battery is either discharged or worn out; or the battery cable clamps and battery posts need to be taken apart and thoroughly cleaned.
If the headlights come on at normal brightness; leave them on while you turn the key to the start position. If the lights then dim way down or go out when the key is turned to start; go to the comments in the previous paragraph about the battery.
If the lights stay bright when the key is turned to the start position; but there is no sound from the starter or engine; the starter solenoid is either defective, or is not receiving a voltage signal from the ignition switch. This is either caused by a defective starter relay, or a defective clutch pedal position switch (only used on manual transmission vehicles), or a defective neutral safety switch (only used on automatic tranmsission vehicles), or a defective ignition switch, or a blown fusible link (which is usually caused when an alternator diode shorts out). If your van has an automatic transmission; try moving the shift lever to Neutral instead of Park, and see if the starter works in that position. Many times the neutral safety switch stops working in Park; but still works in Neutral; because the Neutral contacts are used far less often than the Park contacts.
If the starter turns the engine over; but it does not fire; the timing chain could have broken or jumped out of synch. The camshaft position sensor could have failed; which would cut off the trigger pulses to the fuel injectors. The crankshaft position sensor or the ignition coil could have failed; either of which would shut off the spark. The fuel pump, or fuel pump relay could have gone out, or the fuel pump inertia shutoff switch could have been activated.
There are far too many possibilities here to sort out with scant information and no way to test or inspect parts. If you want to discuss this further; please do not open a new question. Just click the "answer this question" button, type your message in the box that appears, and click the 'submit answer" button. Thank you.