I am having the exact same problem, my wifi isn't the best so i wanted to install OSX from my external (hackintosh) SSD, when trying to do so i also screwed my EFI partition up. (atleast i think so.) it doesn't wanna turn on anymore, a black (although backlit) screen shows, apple logo lights up and the Force Touch trackpad works, Caps lock doesn't though. i tried all the normal boot up modes i could find on the internet and none worked... it's unbelievable that a external HD can **** your $2100,00 macbook pro up in a hitch with no way of turning it back. i went to my local apple store and it's being repaired as i'm typing this. problem is they are installing a whole new logic board, and it most likely won't be cheap. (i'm getting a receipt in 1~3 weeks) but because my display got replaced the day before this happend there is a chance they will fix it for free because i told apple that. (i'm out of normal warranty by the way.)
BUT!!!
if someone knows a easy (definite and not too expensive) fix to my problem, be free to share it with me, because when i get the receipt i have the chance to decline it if it's too expensive for my liking!