This question already has an answer here:

I tried dual booting Windows 8.1 and Ubuntu 14.04. Everything worked fine, and I know that I installed both operating systems on different partitions. Now, it only boots to Ubuntu, and it wont show me the Grub menu to boot into Windows 8.1.

merged by Thomas Ward Apr 16 '18 at 22:13

This question was merged with Unable to boot into Windows after installing Ubuntu, how to fix? because it is an exact duplicate of that question.

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