If you suddenly got some unexpected behavior on your Windows 8 PC, then the first thing you should do is take a diagnostic approach by way of elimination. There are plenty of factors which can be the cause of issues like slowdowns, BSODs, freezes and even sudden reboots. The best way to detect what exactly is the cause of the trouble is to perform a clean boot. Using a clean boot, you can find if the OS is damaged by some third party app or a bad driver. By preventing them from loading, you can exclude the influence of these two factors.
To perform a clean boot, you need to perform two steps.
First, we will check if any third party application is causing the issue. If you disable all third party apps from startup, it will help to eliminate software conflicts.
Press the Win + R shortcut on the keyboard. Type msconfig in the Run dialog and click "OK".
On the 'General' tab, click to select the option Selective startup, and then uncheck the Load startup items check box.
Tip: If you are pretty sure about some app which runs on startup that's safe to run or is essential for your PC, then instead of using Selective startup, you can disable or enable apps individually via the Task Manager in Windows 8.
Open the Task Manager. In the Task Manager window under the Startup tab, right click on each startup item which is enabled and select Disable.
Such diagnostics can be definitely helpful in finding issues related to third-party apps.
The second step is a Safe boot.
You might need to enter Safe Mode for several purposes, i.e. to troubleshoot some issue with drivers. If you system runs perfectly in Safe Mode, then it's a good idea to review your drivers and check for updated versions on Windows Update or from the hardware manufacturer's website.
Please refer to the following tutorial: How to boot into Windows 8.1 safe mode. It is also applicable to Windows 8 RTM.
You can also try enabling the Last Known Good Configuration feature in Windows 8.1 and Windows 8, if the OS isn't booting at all. This starts Windows with the last hardware configuration that booted successfully.