Try jump starting it. A weak battery acts like yours is doing. Maybe you left something turned on that ran the battery low. As long as you've been waiting for an answer, you battery could have rested long enough to self-rejuvenate enough to start the car- that is if the battery isn't so old and decrepit. Have it checked. If you have a ford, make sure the charging system fuse didn't blow out. The alternator, which is good, cannot charge the battery when a ford charging system fuse blows out. They usually sell you a new alternator and battery, then after the new battery runs down, they change the fuse and sell you another battery, or exchange the battery and alternator by warranty- and don't give you any of your $500 dollars back you spent instead of changing a $1 fuse.

Battery, probably. Power required for a stereo is nothing compared to the power needed for an engine start.

Gas is a possibility but battery may be to low to power the starter of the car to turn the engine over you may have to ask someone to help you jumpstart your car

Is it in Park?