My '94 Honda Civic (1.5l VTEC-E) was overheating, and I replaced the head gaskets. Now it turns over but won't start. What should I do?
There are five things I can think of which could possibly be at issue, but as written, your question is very broad and could have many different answers. You don't even give any thought to telling us whether or not the engine will even kick over or if it just cranks over with no other signs of life.
That said ...
Number one: Your cam timing is off.
This video should help you ensure you have the cam timing correct.
Number two: You don't have fuel pressure.
Put a fuel pressure gauge on your fuel rail somewhere and see if you have pressure. You can probably hear the pump prime when you turn the key. If so, this would be at the bottom of my list to check. Especially since the engine was running when you tore it apart.
Number three: You didn't connect power to your ignition system.
Go back over and check your wiring to ensure everything is in place and you didn't inadvertently break something in the process of putting it back together. You can also put a multimeter on your coil power lead to ensure there is power there as well. I doubt the coil itself is the issue. You can also check at the spark plug to ensure you are getting spark.
Number four: You didn't attach your spark plug wires in the correct order.
Carefully ensure you have the spark plug leads back into the correct position. Here is a diagram of how they should be attached:
Number Five: Your distributor may be 180 degrees out.
I'm not sure if this is even a concern, but it might be. If the distributor is 180 degrees out (meaning, the rotor is placed backwards when the distributor was attached to the end of the head or 180 degrees out of position), it will be firing the spark plugs on the exhaust stroke instead of the compression stroke, as it should. You'd need to static time your engine to ensure this is not the case. Again, the lack of engine information is hurting you here. I say might be on this one, because I'm not sure if you can even do this with Honda distributors, but it may be something to eliminate.