Device won't turn on, isn't recognized when plugged in. Help!

[manutastic]

Hi!

So I was following this guide to install TWRP. After flashing the .img and rebooting the phone turned off.
Now it won't turn on at all, and gives no signs of charging or being alive in any way.
When I plug it in I get this message :


I tried following this post to "unbrick" the phone, but I can't install the drivers because I get the message "Windows has determined that the best driver for this device is already installed".

I'm completely lost about what to do next. Please help!

 

[manutastic]

SOLVED: I was able to get the phone into fastboot mode somehow. Not sure how