My fonts have changed and cannot be used - why and how to resolve this?

komhere wrote:

Why have all my fonts .ttf and off. files become exec files and how to resolve this?

From your Font Book.app>File>Restore Standard fonts

Restore fonts that came with your Mac using Font Book

Control (right) - click on one of the problem fonts and select Get Info from the contextual menu. In the Info pane for the file go to the Open With menu and select Font Book. Then click on the Apply to All button. Reboot and see if the problem persists.

Be sure to have a current an full backup of your boot drive before trying any type of fix. Better be safe than sorry in case anything goes sideways.