While I was using my Mini, multiple sets of horizontal lines appeared in the screen and everything froze. I restarted and now I get them on startup. I have reset the PRAM and when I tried to open in Safe Mode, it happened too. I'm at a loss for what to try next. I attached a picture too.
Try resetting the SMC
Intel-based Macs: Resetting the System Management Controller (SMC)
If it has Lion or later try booting to Recovery
OS X: About OS X Recovery
Otherwise boot from install DVD.
If still problem, make an appointment at the Genius Bar of an Apple store since it appears you have a hardware problem.
Apple Retail Store - Genius Bar
It's not a hardware problem, because after update 10.10.3, every Mac with Intel Graphics HD 4000 has the same issue!!! It's obviously a software problem, perhaps a graphic drivers problem. I went to Genius Bar and an Apple genius confirmed that my Mac Mini Late 2012 hasn't any hardware problems!
There are several threads on this forum about this issue, for example:
Cold restarts after 10.10.3 update.
and
Flickering after 10.10.3 upgrade
Apple should fix it. Let's describe this problem to https://bugreport.apple.com/ and open a ticket, so Apple'll try to fix it!
Double horizontal lines on startup of Mac Mini
