If you have a corrupted Segoe UI Symbol font, it can cause an annoyance with the Windows Operating System. Why? Well, the font is used to display icons (such as a power button) on the logon screen and without a stable font, you will see vertical rectangles in its place.
Repairing the Font
To repair the font, you will need to navigate to the fonts folder in the Control Panel (Control Panel\Appearance and Personalization\Fonts) using a local or network account that is a member of the Administrator group and then paste over the corrupted seguisym.ttf font aka (Segoe UI Symbol Regular) with one that is not corrupted.
Operating System Version
Do not worry if the you do not have the same Windows OS available. The troubled PC that I was fixing for a client was Windows 8.1 Professional and I was able to take the font from my Windows 10 Professional and copy it to their computer to fix the problem. All you need to make sure is that the font is not corrupted and is a current or later version.
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A restart of the troubled Windows PC is not required but you will need to logout to see the changes on the logon screen after you have uploaded the font. If you do not have access to another Windows PC, you could try to ask a friend, coworker or search the Internet but make sure it is from a source that you can trust.