Running the command
nuget.exe restore .\MySolution.sln
gives this error:
Unable to find version '1.0.0' of package 'Microsoft.Net.Compilers'.
where the package has previously been installed and restored, but it has somehow become corrupt.
It turns out simply updating the NuGet version via
nuget.exe update -self
to update from 2.8.0 to 3.4.4 was sufficient, and the package now restores correctly.