Hook up test leads to starter solenoid and see if there is the constant 12v on the big wire from the battery. If yes, check if small wire has 12 v when attempting to crank. If yes, you either have a bad starter or bad engine/starter ground connection. You could try tapping on the solenoid to see if it will free up and start (works, but not permanent fix). If no, then you know the problem is elsewhere, and as others have said you could then check your ignition switch, wiring, neutral interlock, etc.
Edit: Take note whether or not the lights dim. I'd expect they don't, as the starter is probably not drawing power. If the engine was completely locked up, there wouldn't be much noise, but the lights would dim and the volt gage would drop when you turn the key (starter drawing power but not turning). That happened to me once, but I was well aware the engine had suffered a serious problem before turning the key- it suddenly stopped running and make a crunch/bang noise. It doesn't sound like that's what happened to you, but I'm just mentioning it in case it's relevant
