2001 Mercury Sable would not start

There's dozens of things that can cause a no-start condition. To start with, you have to determine if you're missing spark, fuel pressure, or both. Diagnostic fault codes need to be read too.

It's important to understand, if you don't already, that brakes and steering do not lock up when the engine stalls. If they did, you'd hear about people running into trees and oncoming traffic every day. You will only loose the power assist and will have to tug harder on the steering wheel and push harder on the brake pedal.

"Not starting" also doesn't tell me what it's doing. Does the starter spin the engine fast enough for starting? If not, suspect the charging system quit and the battery ran down. Try charging the battery at a slow rate for an hour, then see if the engine starts.

Did your mechanic actually diagnose a bad fuel pump or was that an educated guess? GM fuel pumps typically fail while you're driving, leaving you sitting on the side of the road. Ford pumps, like Chrysler pumps, rarely fail while you're driving like you described. When they fail they fail to start up when you start the engine. While the pump could be bad, based on the loss of power while you were driving, the pump would not be my first suspect.

You haven't included anything that can be used to form a plan of diagnosis. You didn't even list which engine you have. "Everything checks out to be fine" doesn't tell me what was checked. Mechanics start with the basics. Is there spark? If not, don't get hung up there. Also check for fuel pressure. If both are missing, it's not a spark problem or a fuel problem. You have to look at what the two systems have in common, which involves the crankshaft position sensor.

When you DO have spark or fuel pressure, the system that's not working is the one to diagnose. The fuel supply system is not monitored, but the ignition system is. That's where the diagnostic fault codes come in Those will indicate the circuit or system that needs further diagnosis.