Installation crashed

[Yeoyakim]

Hello,
I'm on MacOs. I tried to install Ubuntu Touch on Google LG Nexus 5. The process started well but it stopped and the software could no longer detect my smartphone.
My phone is blocked with a black screen in the middle of which is written GOOGLE and below an open padlock.
I can't do anything anymore, neither continue the installation, nor go back.

Do you have an idea ?

Thank you

P.S. : sorry if my problem has already been the subject of a post, I could not find it.