If USB debug is enabled, connect the phone to the computer and run adb.
adb reboot bootloader
fastboot devices
(If it doesn't show anything, wait a few seconds and try again.)
Wipe the user data/cache:
fastboot -w
If USB debug isn't enabled, you have 2 choices, depending on what you'w doing with the hone after you wipe it.
1) Have a local shop plug a screen in temporarily so you can do a factory reset.
2) If you're sending the phone in for repairs, the first thing they do is a factory reset, so don't worry about it. (They're protecting themselves from your claim that they stole your data, by making sure that there's no data to steal.)