Hello autograph and welcome to the HP forums!
You have an Officejet 8500A that is presenting a white screen and multiple flashing lights? I'm sorry to hear that but I will certainly do my best to help :)
I would suggest that you start with plugging the printer directly to a wall outlet to ensure the printer has all the power it needs to function properly.
If it's already plugged to a wall, or that doesn't seem to change anything, please try updating the firmware of the printer from the computer. For Windows firmware, click here and for Mac firmware, click here.
Let me know how this goes for you!