This is known to be caused by disabling Windows Control flow guard (CFG).
To turn this back on:
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Search for and open Windows Defender Security Center on the start menu.
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Go into App & browser control:
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Scroll to the bottom and click on Exploit protection settings:
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Ensure that Control flow guard (CFG) is set to On by default:
This morning my Hyper-V was working, then this evening it wasn't. Disabling CFG was the only noteworthy activity I had done on Windows aside from regular application use. After re-enabling the feature, Hyper-V was working again for me.
I disabled the feature in the first place due to an issue with Windows 10 build 1709 which is severely impacting my TortoiseGit performance in that the diff tool takes about 5 seconds to open.
Another users comment on that page reports the same problem.