what causes an engine to lock up

Most likely oil starvation. If the oil level was low or the pump failed, a rod bearing can weld itself to the crank. Check if you had any oil leaks or if you recently changed oil and might have forgotten to add enough.

Could be a dropped valve or timing chain failure. If the chain snapped, the pistons can hit the valves. Pull the valve cover and check timing marks before you try to rotate the engine by hand.

I had a similar thing with a Polaris. Turned out the starter clutch locked up. Not the engine itself but it felt like a seized motor. Try turning the crank bolt with a socket; if it moves a little, it might be the starter.

Overheating is another common cause. If the coolant was low or the radiator clogged, the piston could expand and seize in the cylinder. Check for signs of coolant loss or white smoke before you tore it down.

Don't forget about water ingestion. If you crossed a deep puddle and sucked water into the intake, hydrostatic lock can bend a rod. Did you hit any water before it locked?

I disagree with the oil starvation being most likely. On these Yamahas, the cam chain tensioner often fails and the chain jumps timing, causing piston-to-valve contact. That'll lock it up tight. Check the cam chain and tensioner first.

Before you do anything major, pull the spark plug and try to turn it over. If it frees up after cranking, it might be hydraulic lock from fuel or oil in the cylinder. That's easier to fix than a seized bearing.

If it seized from oil issues, you'll likely find metal in the oil. Drain the oil and cut the filter open. Chunks of aluminum or copper mean bottom end failure. That's a full rebuild or replacement.

Could also be a broken piston ring or skirt. If a ring breaks, it can jam between the piston and cylinder wall. Pull the head and look at the cylinder walls for scoring. That's what happened on my Raptor.

Check the valve clearances too. If a valve tightened up and stuck open, it could have hit the piston. Unlikely to cause a full lockup, but possible after high RPM. You'll notice if the valve is bent when you remove the head.

I've seen engines lock from a loose flywheel nut. The flywheel shifts and magnets hit the stator. That might make a grinding sound but not a full seize. Still worth checking since it's easy to inspect.

One thing nobody mentioned: transmission issues. If the transmission locks up internally, the engine can't rotate. Try pushing the quad in gear vs neutral. If it rolls in neutral but not in gear, the problem is in the gearbox, not the engine.

Safety first: disconnect the battery and chock the wheels if you start turning the engine over with tools. If the piston is seized and you force it, you could break a rod or punch a hole through the block.

I'm a mechanic. Most used ATVs I see with locked engines have low oil or no oil. Check if the dipstick even shows oil. If not, you're looking at a seized crank bearing. Time to price a used motor on eBay.

Don't rule out a failed oil pump. Even with enough oil in the sump, if the pump gear stripped, you'll have no pressure. Symptoms are sudden knock then lock. Pretty rare but happens on older models.

If you want to save money, pull the engine yourself and take it to a machine shop. They can tell you if it's rebuildable. But be realistic: a 2007 Grizzly might not be worth a full rebuild unless you have sentimental value.

Could also be a thrown rod. That's dramatic, usually accompanied by a loud bang and hole in the case. You'd know if you saw oil spraying everywhere. If not, probably not a rod.

I think you should check the simplest things first: battery condition and starter? A dead battery might make it seem locked. Jump it with a good battery and see if the starter tries to engage. If it clicks, it's locked; if it spins, it was just weak battery.

Water in the oil can cause an emulsion that doesn't lubricate. If you ever washed the quad with a pressure washer, water might have gotten into the case through the dipstick or breather. That can cause a sudden seize too.

Before you spend money, ask around local ATV clubs. Someone might have a spare engine for cheap. And when you find the cause, post back here so we all learn. Good luck.