stuck on google logo

my nexus 5 prompted me to do a system upgrade. I said yes, while upgrading, about 80% along the way, it suddenly showed the small android logo with an error message underneath. so I restarted it by pressing the power button and since then its stuck on the google logo. it stays on it for hours. the phone is practically bricked.

I did hard reset to factory setting. I did cache wipe. nothing changed it stops at google logo and that's it.

what is the solution to this problem?

Simon