2000 mercury mountineer won't start

The same thing happened to me many years ago. It was not out of gas but it was so low that the gas wasn't reaching the fuel pick up hose in the tank, at that particular angle.
I coasted down the hill to level ground and it started easily.
Try putting some gas in the car and try starting again on the hill. If it still won't start then something else is wrong, but I have have no idea what it could be.
Good luck

Relay, on top of fuel tank under parcel shelf, check its getting power, if not its fuse, if it is, bin it get a new one.

If there is water in the fuel tank, when the tank gets low, this happens. Get an addtive called HEET and add it to the tank

Does the vehicle gets spark? if yes check with scanner whether is there any problem with crank or cam sensor failure.

they had a problem with the fuel pressure regulator and when cranking slow the intake manifold blows up there is a recall on some years

Fuel in the tank does not mean fuel in the injectors. Run a fuel pressure test to see how much fuel pressure there is. Also check the injector pulse with a noide test light. The injectors may not be getting a ground to fire because of a bad fuse.

check fuel pump,fuel pressure regulator

That is typically indicative of a fuel pump issue. It isn't necessarily the fuel pump itself, but the way it is positioned in the tank when low on fuel can keep it from getting fuel.
If you fill the tank and find however that the problem still exists then the other cause is related to parking.
Your Buick has a safety mechanism that will not allow it to be started if not fully parked, called the Brake Transmission Shift Interlock. Not only does this solenoid controlled component not allow you to shift out of park without applying the brake, you also will not be able to start it if this mechanism is faulty, or something in the gear select linkage prevents the shift selector from properly engaging the parked position. When on a hill and you shift into park, the vehicle weight is rolled against a component of this system, preventing it from properly "registering" the parked gear select position.
What you can do the next time you need to park on a hill is with your brake applied and the gear select in Drive, apply the emergency brake and hold it down, THEN shift into park. Then release your feet from the brakes. This should help.

Hi, if an engine cranks but refuses to start, it lacks ignition, fuel or compression. When the engine cranks normally but won't start, you need to check ignition, fuel and compression. Ignition is easy enough to check with a spark tester or by positioning a plug wire near a good ground. No spark? The most likely causes would be a failed ignition module, distributor pickup or crankshaft position (CKP) sensor.

how low was the fuel if it is on the reserve tank then you may have to roll the car to level ground the fuel pickup may not be able to **** the gas up being that it is so low the gas may all be to the rear of the tank the pickup is usually in the middle of the tank once on level ground it should start if not you may have to add some fuel to the tank. as a word of advice try not to let yor tank get to the point where the fuel light comes on because your tank has moisture and trash in it from the gas and whenever it gets low like that it can **** it up and clog your fuel filter or even your injectors. hope this gets you going .let me know how it turns out.