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There is a known problem (with an as yet unknown solution) that is with Sony to fix (that much about the problem is known) that corrupts the running software and causes this to happen.
The workaround is to Factory Reset the TV, which puts the software back how it should be, and restores the functionality of the YouView Guide.
But how long it is before the problem reassert itself depends on when the Sony software takes the unhappy path once again, and reimposes the software corruption causing this issue.
Could be months, could be minutes.