try to inspect your your lock and tumbler or key ignition switch.
Follow the + battery cable from battery should connect to starter solenoid (relay)
A Montero may not start if the battery is dead or power is not reaching the starter. Both can be readily diagnosed by a local mechanic or dealership.
how do you replace the starter on a 2001 montero sport?
check park / neutral safety switch, if standard trans check the clutch safety switch
Check the battery cables or starter.
How to change a starter on a 1998 montero sport
check your battery, maybe it's time to replace it
Dead battery? Loose or dirty battery cables? Bad starter solenoid? Bad starter? Neutral switch?
I have just came up on this same problem with my 2000 Kia. Turns out it was the Ignition switch. $160.00 to have it replaced.
battery needs replacing, make sure you replace it with a battery of correct cranking amps
Voltage Regulator?
Double check the battery cables at the battery and at the engine, be sure they are clean and tight.