Today my computer screen went of centered so half my screen is black how do i fix it?

my friend was playing a game today on the computer when he was done he turned of my moniter. when i turned it on the startbar was in the middle of the screen and the left side was completely black. The right of my screen was completely cut out. Someone please help me make it normal again. Also its an older moniter
Update: thanks for the fast answer!!!! it worked perfectly! and it only took 10 minutes for the answer!
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