Q: macbook pro mid 2012 won't start up

Take it in for repair.

Did you start with "internet recovery" Computers that can be upgraded to use OS X Internet Recovery - Apple Support? (Should link to this page OS X: About OS X Recovery - Apple Support)

If it does not want to do that, you can force the system to "detect setting" with an SMC reset Resetting the System Management Controller (SMC) on your Mac - Apple Support and try internet recovery again.

Then do as Kappy advised -- take it in for repair.  I gave those links just in case they would help.

Good luck.