Charge your battery and try to force it into fast boot?
Give it a few hours, if not when you do get into fastboot and try to flash the files it will give you a "low battery" warning and will not start the process until a sufficient charge is provided
OP might need a separate charger or a temp battery switch if he flashed a rom. I'm pretty sure the charging-while-off feature is blur-only. I had a problem like this trying to get CM4GB on my D2 when it was in leak stages. Had to borrow my GFs D2G battery to get some life in it so it would take the SBF.
I am using a mac and connected by USB to bionic in Fastboot mode. Looks OK but gets stuck at the message "waiting for correct USB state." What do I do know? The phone is in a bootloop, so I have no more control over the USB state. Phone says USB connected. Help please?