How do I remove Grid patern from monitor

Screen shot would help, however I suspect this is a hardware issue.

GPU, LED/LCD, cable or cable connection issue.

'Apple Service Diagnostics' test

Genius reservation http://www.apple.com/retail/geniusbar/

on-line https://getsupport.apple.com/GetproductgroupList.action

check warranty https://selfsolve.apple.com/agreementWarrantyDynamic.do

Are you using an apple monitor or 3rd party?

if apple see this document:

https://support.apple.com/en-us/HT201177

if 3rd party

see the manufactures website for resetting the monitor if the front panel does not have a help section, set the monitor back to defaults.

If the problem persists try a different power outlet or change out the cable. This is not behavior inherent to OS X or Mac Mini's.