"Computer gets stuck starting up the operating system."

1.Do you get error messages or nothing? If you do get error messages write them down.
2.When did it last work?
3.What was the last thing you did before shutting it down?
4.Did you install new software, or hardware? Did you delete any files?
5.For Mac, try starting system with extensions off (you do this by restarting while holding down "shift" key). Then turn extensions on with extension manager slowly to find problem.
6.In Windows press "F8" at start up, and try launching in "Safe Mode".