I'm sure several factors could have corrupted the license file. I am mostly pointing this out as something for users to watch out for.

This happened in windows 7 with updates disabled.
Antivirus was also disabled.
I'm not sure if windows defender is proactive enough to cause this issue.
No recent installs of new software or updates happened anywhere near the time of corruption.
Only 1 version of sketchup is present on the PC.

The grayed menu appears to be an issue if multiple versions of sketchup are present, though. From the help center:

"Tip: Is the Add License area grayed out and inaccessible? This means you have installed SketchUp Make instead of the SketchUp Pro trial. If having two versions of SketchUp on your computer might be causing problems, try uninstalling SketchUp and then installing only the version you need."

Like the title says, my license file was mysteriously "unreadable" today. My office uses sketchup 8 pro, and we have been very happy with that version with no need to upgrade in sight.

Well, today, opening sketchup gave me an error message that the license file couldn't be read. I went to the file which now had 0kb instead of 4kb. Within the sketchup help menu, all licensing options are now greyed out. Oh perfect. A corrupt license file and no options to appoint a new one within the program.

Fortunately my server was able to roll back the file to a previous version - the original install. ...And now sketchup pro works fine again. Licensing options are no longer greyed out.

Of course this could all just be due to a corrupt file. Or maybe sketchup is trying to force my hand into an upgrade that I do not want or need. With the licensing options removed after the corruption, the situation seems a little fishy.

Either way, I just wanted to give a heads up to the legacy users out there that this might be a potential issue. Back up and encrypt your license file for future use! This could have been catastrophic for workflow without a roll-back.