In some rare cases, especially after making changes to the monitor setup (amount of monitors attached to the PC, switching the main and secondary monitor, changing resolutions, etc.), the game may start minimized and can't be maximized. A workaround in case this happens is the following:
In Steam, right-click on Portal Knights in the games library
In the tab Local Files, select the option Browse Local Files...
The opened Explorer window contains a file called portal_knights.cfg
Deleting this file and relaunching the game will reset the graphical settings, enabling the game to auto-detect a correct setup for startup.