13'' MBP Mid 2012 not turning on, just one LED blink

[gonzija]

Hi MacRumors!

I'm having this issue with my MBP. After installing macOS Sierra on my external HDD, it rebooted and it came up with this.
Does anyone know what to do? Thanks!

 

[Fishrrman]

I wonder if this could have anything to do with a "failed firmware update" ...?

A trip to a brick-n-mortar Apple Store might be needed.

 

[Audit13]

What OS was running on the machine before installing Sierra?

Tried pram and smc reset?