Lenovo y50k dual boot win 10 and win 8.1 issue?

I have lenovo y50 4k laptop with 1tb hdd, 16gb ram and 4gb nvidia 860 gtx.
I attempted to do a dual boot with win 8.1 and win 10. Initially it worked well and fine up to 2 months. Suddenly it started a slow boot for win 8.1 and kept on. I googled almost everything but couldn t find any solution.
Any help would be appreciated.
Thank you:-)

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