2000 Hyundai Elantra won't start

I have a 2000 Hyundai Elantra that will not start. A week ago today, on a Sunday, was the last time I drove it. That day I had a new after market cd player installed. As this car is not our only vehicle it was not driven for a few days. On Wednesday I started the car but after realizing it was low on gas I decided to take another vehicle as I didn't have time to stop and get gas. The next day, Thursday, the car would not start. The CD player comes on as well as the usual lights that come on upon turning the key. The car does act like it is trying to start and makes noise upon turning the key however it won't catch. I did go and fill a gas can and put in about 4 gallongs of gas and the car still will not start. Because of the cold weather it was considered that what little gas was in the tank could have frozen however as the temperatures have warmed it still will not start. I did also notice that the check engine light stays on even when all the other lights have gone off but I'm not sure if that is reliable since the car is not actually started.