could be many things from a bad starter relay, bad transmission range selector, bad IGN switch, etc. If there's no power, Check 50A #12 fuse in in underhood junction box. Also check fuse 10A #29 in fuse box left side of dash. If they're good, check for power on the yellow/light blue it's time to break out the wiring diagrams and check for PCM ground at the starter relay and good signal from the transmission range selector.

If you DO get power at the starter motor yellow/light blue, and the battery is putting out at least 12.5 volts, then the starter is bad

Have the 12 volt battery *load tested at ant store other than Auto Zone where new ones are sold. A free 15 second battery load test will tell you the reserve amperage capacity of the battery. Amperage spins starter motors not voltage.

It could be your battery man, alot of the time people mistaken a faulty battery for a faulty starter motor,

Could also be the neutral safety switch. Try to start in neutral, not park.

I have a 2003 Mercury Mountaineer, it runs fine. I drove it to the park and back with no issues a few weeks ago, but when I tried to drive it again that same night it would not start. When I turn the ignition it doesn't make any sounds, no grinding, no clicking, just the gauges move and the lights function. I'm thinking the starter, but am open to suggestions and advice since I only have experience with testing/changing the starter on a lawn tractor.