Remove the battery and AC adapter. Press and hold the power on button for 10 seconds. Then put the battery back in and attach the AC adapter and try to start. If this fails zap the PRAM, here's how: http://support.apple.com/kb/ht1379

First, it's dangerous to use a computer without battery, cause when the AC adaptater is unplugged , the machine suddently shuts down, and this is not recommanded for computer health ( It can damage files, hard driver, or something else) . So don't do it again.

According to what you tell us , I can say that there is 2 possibles causes:

1-Your files has been damaged, so that Mac won't boot

2-Your RAM Memories can be damaged too.

Try this , and tell us more about after doing them:

1-remove the RAM memories and try each one of them separately (plug one and try , then plug the other and remove the first one and try again.

2-At Start up , press "Commande+S" to boot in single user mode and type

"/sbin/fsck -fy" exactly like this ( there is a space between "k" and the dash.

Do it until you get this message : The volume _name_appears to be OK!

and type "reboot" to restart the mac.

3-if it won't boot, you will have to reinstall it .

4-Force mac start up holding "Commande+control+powerbutton"

If all this have help you, post us back and I'll suggest you more solutions.