OCCT is a powerful and free (for personal use) PC monitor and stability checking tool.
Launching the program on our test PC immediately displayed useful PC and GPU-related system information, alongside with a series of graphs plotting how our system temperature, CPU voltages, RAM usage and other details are varying over time.
And if these don't in themselves reveal any problems, then OCCT can launch tests to exercise your CPU, GPU or power supply, again allowing you to see in detail how your system's temperature, voltage and other details might respond.
If you've just been overclocking your system then running a tool such as OCCT can be very useful, as it will help to confirm that your PC can still cope when it's overloaded.
Of course, if it can't now cope then this may be an issue. Stress-testing a computer in this way, perhaps overheating it, can cause real damage. If you watch what's happening, though, this need not be a significant problem. And better still, OCCT will in many cases stop testing if particular measured values are too high (and you can configure these settings for yourself).