Help! 92 civic won't start

I disconnected battery 3 weeks ago, and went away for a trip.
After I came back, i connected the battery, and tried to start the car.

The starter works well, and cracks the engine again and again, but
it won't start. I keep the key on "on" position, the lights of gas and battery stay on, while others off.

I check the gauge of gas. It is still enough. And I check the voltage
of battery, it is 12.4V when the engine is off. Since it is rainning
recently, I open the distributor cap, and clean the nodes and rotors.
However, the engine still won't start.

Any suggestion? Thanks in advance.

Can you hear the fuel pump kicking on when the key is turned "on" position?Have you checked to make sure you are getting spark at the distributor?
Hope you get it goin....

I can hear the sound of fuel pump below the rear seat. But I don't know how to check sparks.

However after I put more gas into the gas tank, and pressed hard on fuel pedal when I started,
the engine started, and now everything is OK.