Which game? Have you tried the key combination of alt + Enter?
PANZER GENERAL 2 and yes i have tried alt enter
is not a problem with the game i have somehow upset how windows works by default and i have no idea how to fix it .game runs in a small borderless window top left and wont resize with atl + enter and its not just panzer 2 thats messed up a few of my older games are the same. new games are all working fine
Try this solution from Microsoft.
Steps To Run Games In Full Screen Mode On Windows 10
Launch the game’s EXE. Make sure you are running the game just the way you normally do, even if the screen display comes out to be wrong. It is likely that you will see the game pinned to the task bar, and in a minimized window. This is completely to be expected, and should not hinder you from performing the next steps.
Right-click the task bar to open task manager, or use the trusty, old CTRL+ALT+DEL command.
Go to the ‘Applications’ tab in task manager and locate the entry for the game you ran in step 1. Right-click this entry.
From the resulting menu, choose the ‘Go to process’ option.
On the game’s process, hit the ‘Switch To’ option.
Select full screen mode, if that option presents itself.