It's probably a faulty start-up program, or a messed up bootskin. I'm not exactly sure how to fix the problem if it is a bootskin, but if it's a start-up problem, try booting in safe mode, and disable all optional start-up programs. Try booting up normally after that, and if it works, that confirms that it's a start-up program that's causing the problem. To fix it, you should enable each program one by one, and start up the computer with each different start-up configuration. As soon as it crashes, you should know which program is causing the crash, and you can just uninstall it to fix the problem.
Sorry if the explanation of what to do is a little unclear, but I'm not that good at giving explanations. Anyways, hope I helped!
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