Could it be that you are missing gmapprom.img file in the memory of the Garmin Nuvi 855.

I have run into a user that did strange things by deleting the gmapprom.img and nothing would work again.

They could not see the all Garmin folders on their PC.

By using a file brower called XYplorer it can see all the folders on a nuvi.

It is far better than File Explorer for lots of things.

You can download a trial version of it.

by seeing that the gmapprom.img was missing he was able to copy over a version of it.

restarted the nuvi and it was up and running again.