2nd monitor stopped working
My 2nd monitor has worked for years, but now it randomly fails. I can get it back by restarting the iMac. Suggestions. I am suing Mohave version 10.14.5
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First, try some resets to see if that solves the problem:
https://support.apple.com/en-us/HT204063 NVRAM Reset
https://support.apple.com/en-us/HT201262 SAFE Mode reset
If neither work, then replace the cables and if those don't work then it's likely the external display.