Just a heads up, ignore Dell Diagnostics. Also, if you're using a copy of Windows installed by Dell, expect bugs. They are notorious for bloatware and useless OS changes that tend to cause problems. Their whole business motto is using proprietary hardware and modified software to prevent you from going anywhere, so when your stuff breaks, you're forced to go back to them and spend more. That is how they make their money back from their systems being so cheap.
As was suggested, you're best off with reinstalling fresh. Make sure you back everything up you wish to keep, and then format your HDD and start all over. If you can, install a version of Windows 8.1 that is not from a Dell disc. That way you are getting a standard copy of Windows that Dell has not screwed around with.
Once you get everything working again, immediately re-activate your copy of Windows, and get your updates fully done. If you run into the issue again, check your hardware as Teddy mentioned. Dell's hardware is also very well known for breaking just after its warranty date, for the same reasons I mentioned in the other paragraph of this reply.