René Logeais 

Where has this been asked before?  It would be helpful to see what has been tried... if anything... to resolve it.

Have you tried the obvious... restart your computer?  It it happened "all of a sudden" that would be my goto solution as I do see odd behaviours from time to time with many applications, not only Studio.  Being "forced" to move to office 365 created more "odd" problems on my machine than I've ever seen!

Have you looked in the keyboard shortcuts section of Options to see whether it's still shown there? No reason why it would suddenly disappear from there, but still... 

René Logeais 

In the absence of you responding I wonder if you just selected the "Trados" profile as opposed to the "SDLX" or "Default"?  I know you said "all of a sudden" but maybe you changed the profile? Differences are these.

Trados profile

SDLX Profile

Default Profile

The only difference is the very keyboard shortcut you are using.   You can check under File -> Setup -> Manage User Profiles

René Logeais 

Or maybe.... you just opened a document that was all confirmed already so the only shortcut that will work in this case is the Ctrl+Alt+Enter?