You changed an option, most likely a video setting that your hardware wont work with. EQ's detection of what hardware is supported is splotchy at best IMO. If you change a setting that the video doesnt support it will most likely crash. This sounds to be your problem from what little you have said.
Go into your EQClient.ini file and change settings back by hand (not recommented if you dont know what they are)
OR
Delete your EQClient.ini File, EQ will make a new default one for you.
OR
If you can start EQ to get to the Login Screen, click on the Options and reset your Video stuff. You will need to do this if you delete your ini file as well if memory serves me correctly.
But this is a guess....