I have a feeling you can't locate it with iCloud.com until the phone is turned back on. However it seems that if you do go to iCloud.com you can go to Help then Find My iPhone and select things for the phone to do when or if it is turned on. I just tested the "Play a sound" option. I turned my phone OFF and then requested iCloud.com to Play a sound on my phone. Since the phone was off it didn't play a sound. I then turned the phone on and very shortly after the phone was finished booting up it played a sound.
I suspect if you select the "Lock and track" or "Erase" (since your phone is apparently stolen) options that the next time the phone is turned on it will perform one or both those functions. I don't know if you have to stay logged in to iCloud for this to work. You'll have to look into that. Also I suspect this only works if you have previously turned the "Find My IPhone" option to ON in Settings - iCloud (settings) on the phone.