First you need to know how to use the radio to ask for that help. Just putting the headset on isn't enough; finding the microphone button is half the battle, and then you need to know what frequency to tune to, and how. Then you need to follow along with those instructions.
Even assuming everything goes perfectly up to that point, the plane can only /land/ itself -- assuming you're approaching an ILS autoland-capable runway, in a plane with autoland equipment installed. You still have to idle the engines on touchdown and engage reverse thrust. You may have to keep the aircraft on the runway centreline after touchdown. Then you need to shut down the engines so emergency vehicles can approach and passengers can evacuate safely.
In addition to that, you also need to know how to:
Read the instruments. This is non-trivial with modern glass cockpits as the readings on the screen don't necessarily have labels.
Set the flaps at the right time. Too soon and you'll tear the wings off or run out of fuel before you make it to the runway. Too late and you will be flying too fast to land.
Lower the gear, again at the right time to prevent them being torn off by airflow or not being down when you hit the runway.
Know that, once on the ground, the nose gear is steered with the footpedals and not the sidestick (Airbus) or yolk (Boeing).
Set the autobrake so you don't need to brake manually at touchdown. If you get this right, you don't need to mess with reverse thrust.
It would be helpful to know how to turn off the cabin pressurization system so no one is blown out the doors when they are opened.
Even if you're lucky enough to get all this right, you're only going to make a smoking hole somewhere near the runway if there's any significant crosswind.
I think finding the microphone button without messing anything up would indeed be the main challenge. AFAIK on some planes it is unmarked, and has a similar-looking button, also unmarked, on the other side of the yoke. That other button is the autopilot disconnect aka "everyone is going to die" button. Also, if you kick the yoke while trying to get into the seat, the autopilot will disconnect...
Once you find the radio and manage to reach anyone, I'd assume they can get someone to relay information to you on that frequency. At that point it becomes a matter of them being able to give sufficiently idiot-proof instructions and you following them, i.e. as long as whatever bad thing happened with at least a couple hours of flight time remaining and you don't mentally shut down, I'd say it should be recoverable.
It'd definitely help if whoever is in the cockpit has played some flight sim before, because "gear down" is a lot faster to convey than describing what the gear lever looks like and making sure they actually found that one instead of something completely different that will crash the plane.
Mhm. In some planes there's VOX capability (so you can speak and it will transmit for you), in others there's a microphone button on the yoke, and in yet others (like the Boeing 777, which is what I have the most experience with) it's a button on the console in front of you [1].
And assuming you figure out how to use the radio at all, that's still of no help whatsoever unless you happen to know how to tune to 121.5 MHz (the international aircraft emergency frequency) or are still in range of the previous controllers!
[1] https://live.staticflickr.com/7443/27038444724_18cc73d82c_b.... In the right of this image, there is an empty pencil holder on the far right pillar (like the A-pillar in your car). Immediately to the left of that is a set of 2 black buttons and a white dial. The microphone button is the left black one. It only has "MIC" written above it. The unmarked single button on the yoke is indeed the autopilot disconnect.
EDIT: I should also add that the microphone button doesn't mean you'll necessarily be talking on the radio either. The headset also serves as a PA system and an interphone, and it can use the satphone too. This is all controlled on the radio panel next to your leg.
Unfortunately this is typical "All you need is..." fallacy.
Even assuming for a moment that yes, all you need to do is dial in the numbers...
Have you ever tried to run tech support over a dodgy phone connection to someone who's never touched technology before? Where one wrong button push turns off all of that automation? Where there's no chance to go "Ooops, undo that!"? Where it's a very high stress situation, involving a huge number of acronyms?
Even just managing radios could be a major problem - you may quickly find yourself out of range of the controllers, and if you haven't figured out how to ask for and change frequency, well you're now not talking to anyone.
I thought modern airliners can auto-land? All you need is someone to walk you through dialing in the numbers.