The other day I was driving down the 710 I-CA at around 80mph but as soon as I got on the east ramp for the 105 fwy I lost total power as the engine had just cut off and my check engine, battery, and oil lights went on so I came down the ramp in neutral. As soon as I stopped I popped the hood and saw that everything was ok I tried to restart the engine but it didn’t work. At the time the starter would crank the engine so I thought that the alternator was the problem. I had it towed home and changed the alternator but now when I try to start it all I get from it is a single crack from the starter and no crank. I tried to move the engine by hand and it wouldn’t budge. I opened the engine thinking if could be that the timing chain broke but it is completely intact. What do you guys think I could be? I’d appreciate any help at all. My car is an 04 Toyota Matrix XR with a 1ZZ-FE engine if that helps. Thanks!
You say “starter would crank”.
“starter would crank” means the battery and alternator are OK at that point. So I don’t understand why you would replace the alternator.
I suspect that in changing the alternator you accidentally shorted or opened a connection, which may be why it would not crank now. First you have to fix the electrical problem, then work on why it won’t start.
What exactly do you mean “tried to move the engine by hand” ?
From what you seem to be saying it sounds to me that the engine has frozen. If you can put a wrench on the bolt for crankshaft sprocket and can’t move the engine then you have a serious engine problem.
Have you checked the engine oil level . . . ?!
If it seized, due to lack of oil, that engine is a paperweight
And please define "tried to move the engine by hand’
Oil levels were fine, and it had new oil
Yes, this is exactly what’s been going on. I tried to move the crankshaft but it wouldn’t turn
Remove the spark plugs to see if there’s any coolant in the cylinders.
Tester
By moving the engine I meant turning the crankshaft manually
You don’t turn the crankshaft with a wrench
You use a 1/2" drive breaker bar, extension, and the appropriate sized socket on the pulley bolt
Works better than a wrench
Hopefully not an open end wrench, either 
Remove spark plugs as posted above, check for anything unusual in the cylinders, if nothing found, try rotating the engine using a breaker bar/socket on the crankshaft bolt. With the spark plugs removed you won’t be fighting the engine compression. Make sure you are turning the engine the correct direction btw. And that the transmission is in neutral.
If it turns ok then, with the spark plugs removed, my guess your alternator was indeed bad and the battery discharged while you were driving. Eventually there wasn’t enough juice left to fire the spark plugs and run the fuel pump, so you stalled out. Replacing the alternator won’t automatically fix this problem, b/c even though the new alternator is presumably ok, the battery is still discharged. Try re-charging the battery w/a battery charger, the 2 amp rate overnight is better for the battery than a quick charge.