New iMac not starting up

hey guys, bought new imac 2 weeks ago, today tried faster magic mouse speed , did youtube video guy clicked on terminal : defaults write com.apple.scaling 8.0 (not sure if has thought) 10 minutes after computer got frozen , restated it, wouldn't boot. instead white screen folder , question mark blinking.

help?

 

try booting while holding option key. happens then?

 

Forums Macs Desktops iMac

• iPhone
• Mac OS & System Software
• iPad
• Apple Watch
• Notebooks
• iTunes
• Apple ID
• iCloud
• Desktop Computers
• Apple Music
• Professional Applications
• iPod
• iWork
• Apple TV
• iLife
• Wireless