Mriad Pro semi bold not working in latest version of illustrator

[#1]

I just tried to update a (not so old) job for a client and found that the artwork was out due to the Myriad Semi Bold font not working. The bold works and so does the others but not the semi bold. I find this frustrating even though I expect some changes in using older artowkr but not as drastic as this. The semi bold font met with the design and now I have to either use the regular font or bold!

Can anyone explain why the semi bold font doesnt work and if there is a fix?

Thank you in advance.

[#2]

Can you please explain exactly what is happening?

You cannot select the font?

The font doesn't appear n the menu?

The font is reported missing when you open a document?

Also: which version, which system, do you use font  management?

[#3]

Is the semibold installed? Is it an older PostScript Type 1 version.

Did you try to replace it by the Myriad Pro version from Adobe Fonts (Typekit)?