my iphone wont charge and shows that stupid red battery and lightning bolt, what should i do?

it seemed to be working fine just yesterday and now its acting retarded. ive been googling and checking out fixes and ive read something about a restart by holding down the power on/off and the menu button, but i only got it to work once. it came on for 10 seconds and then told me i had less than 20% battery life. then it went back to the lightning bolt and red battery screen. i also read something about taking out the sim card, i read something about rebooting my mac, i must also say… my iphone has been dropped… MONTHS AGO… and it hasnt been having any problems since… my screen is cracked but it still shows fine. i also gotta say, i dont have a wall charger, only the computer usb cable, im going to go to the att store tomorrow and ask them to plug it into a wall charger, if it starts charging then i guess i need to buy a regular charger… if not… what do you think is the problem?