Bad RAM is one thing that causes them. Files—particularly ones you frequently access and save—seem to be inexplicably corrupted. RAM issues can lead to this problem, which can worsen over time. The file structure of your hard drive may slowly degenerate, and you will no longer be able to boot your machine.
What happens when RAM is faulty?
Random crashes or reboots. Crashing during heavy memory use tasks, such as gaming, Photoshop etc. Distorted graphics on your computer screen. Failure to boot (or turn on), and/or repeated long beeps.