Hello & Welcome to Just Answer,
Sorry to hear of any trouble you may be having with your device.
May I ask:
1) Can you confirm that this is a Garmin Nuvi 2555 device?
2) Can you tell me the AGE of the device?
3) Can you tell me MORE about "every way I know to turn it on"? For example, what ways did you try, besides connecting to the computer?
OK - when you say "using the vehicle power source", does that mean that you just "plug it in" and normally the screen comes on (without any other action)?
Also, do you leave it connected to power - to charge the internal battery - when it is NOT in use?
Did we get cut off (I didn't see a response back here to my last questions)?
Let me send the following information - in case I get cut off or have to log out for the night.
Many times these devices get "stuck" in a "sleep mode" and need a soft reset OR a "long power switch" sequence.
To do that, remove the power from the device and then push and HOLD the power switch - for UP TO 2 minutes - or until you see the screen come on.
If the screen does come on, release the power switch and allow the device to fully power up.
NOTE: If you get a prompt asking to power OFF the device, respond by touching YES, then WAIT about 15 seconds and then push and HOLD the power switch again - until the screen comes back on (and let it power up at that point).
If none of this works, there may not be enough charge in the internal battery, and I would recommend connecting the device to the vehicle power adapter for about 30 minutes (to get more charge), and then try to power ON the device by pushing and HOLDING the power switch again as before.
Please give these things a try and let me know your results - when you can. There are some other things that can be checked - if needed.
Thanks, ***** *****