If you have disabled the launcher, you can enable it again in the game options. If you are unable to start the game, you can force the game to show the launcher by using a launch command option:
-
Steam
Right click the game in your Steam Library, go to properties and add -launcher in the Launch Options field.
-
Epic Games Store
Click your profile, select settings and scroll down to Ghost of Tsushima Director's Cut. Tick the box for Additional Command Line Arguments and add -launcher in the field below.