MacBook cannot recognize battery

When I followed the battery replacement guide on this site exactly, I booted up my MacBook to see that my battery had an x through it with the no battery availible popping up when I scrolled over it. I (stupidly) left the computer on while I browsed for a solution. The whole time my charger showed a green light. Suddenly the computer turned off and hasn't been able to be turned back on since (about 2 weeks) despite switching to another battery and another charger. The charger still shows a green light, which means the battery is either fully charged or installed incorrectly. I don't understand how it can be installed incorrectly when the only part to put it into the computer is the plug in. Please someone help me, I have no idea how to fix it and have made multiple forums on here asking for help, to no avail.