Windows 10 Product Key Activation Issue

[Heydon2015]

I rolled back my PC to windows 8.1, however my PC kept displaying blue screen of death carried on with installing windows 8.1 updates and when it restated it took forever to boot the restarting screen appeared to be displaying forever , I assumed wrongly that my system had crashed so pressed the power off button , as ther is no reset button on my PC , and then decided to try and refresh , however when I boot from the sub to refresh it come up with the blue screen of death , and then I try to diagnose problems , and again the blue screen of death ......I just hope I haven't corrupted my hard drive . Would appreciate some help please