2004 mercury mountneer turn ignition panel lights up but engine wont start?

1 With your headlights on before inserting key Do your headlights shine OK ?
2 Do they Dim or go out when you turn the key?.
3. Do you get any clicking sound when you turn the key??

Results
1 if not possible bad battery - or battery cable to battery, engine and ground points connections bad/need cleaning possibly.

2 If they go Out - See number 1 . Also, Started may be enganged t engine and stuck.

3. No Clicking = #1 possible.