No question - it will destroyrunning the engine will break it.
If you're "unlucky", there's enough clearance between the piston and the valves to fit the spark plug. In that case you'll have the spark plug rattling around inside the cylinder. In approximate order, this will first destroy the valves which will not seal properly when their edges and seat faces get damaged, then the cylinder walls, then the piston rings as they're damaged by the gouged-out cylinder walls. The cylinder head and piston will also be damaged beyond repair, but they probably will not immediately cause the engine to fail. After that, the crushed-up ceramic will run through the oil and scratch up everything that gets oil and needs to be kept smooth and clean. The engine may run for a short period, but it will be permanently damaged from. After you've turned the moment you turn itengine on, your next step is to book your garage to fit a complete new engine. Total cost in the UK is probably £2000 for a recon engine and the time.
If you're "lucky", the rising piston and descending valves will hit the spark plug. This will immediately destroy the valves and seize the engine. The reason you're lucky here is that this is repairable because it happens immediately. There's some risk to the piston and cylinder head, but you stand somea fair chance of getting away with itwithout damaging them. Total cost is probably around £500-1000 for taking the head off and fitting new valves.
I've no idea what you're talking about when you say it "fell through the valve". The spark plug doesn't go anywhere that it can fall through a valve, because that's not how an engine works. The valves cover one set of holes into the cylinder (carrying air and fuel), and the spark plugs go in completely separate holes (which are sealed by the plugs). I don't mean this to sound insulting, but I'm not sure you know enough about engines to make an informed assessment of what's gone wrong and how to fix it.
If you have breakdown/recovery cover, call them and get them to trailer the car to a garage. If you don't, call round the local garages to find one who'll pick it up. You might not need a new engine, but you certainly do need them to take the cylinder head off to retrieve the broken spark plug. I'd expect to pay around £500 for that.
A garage should easily do it in a day. It's generally a whole-weekend job for a very competent amateur mechanic with a good set of tools and plenty of experience. (That's how long it took me when I had to change a cylinder head, anyway.) Actually taking the head off is really the easy bit - what takes most of the time is disconnecting everything else around it. This does need skill and care though, otherwise you can permanently wreck the engine. Under the circumstances, I would not recommend you attempt this, unless you intend to scrap the car anyway and you fancy having a try to develop your mechanic skills.