Your OS and hardware information could offer clues. How old is the system, what do you use it for, etc. One thing usually suggested for sudden failures that have workarounds is to use system restore to go backward in time to when you knew it was working. From there you escalate to other measures but I don't think it's wise to just throw possible solutions at something. Work methodically.
So i turned on my computer today and after putting in my password I get nothing but a black screen and my cursor. I have yet to find a way to fix this only work arounds. Everything works fine one I open task manager and stop explorer.exe and start a new one. I've tired safe mode and it started right up. Gonna keep trying everything I can with my last option to do a repair and hopefully not lose any of my data.