I have the same problem as OP with my wife's Intuvia.
This is such an "own goal" by Bosch. They've somehow designed an Achilles Heel into the ebike system that didn't need to exist at all.
Our Intuvia displays have a small rechargeable battery in them, and this battery needs to recharge from time to time, drawing power from the main 48v battery. Why does the display have a battery at all? I guess that's because some people might want to use the bike computer features of the Intuvia away from their bike. As in, take it to their desk and look over their odometer. What a useless feature!!! But because of that, it has a battery, and they engineered the entire bike system to fail completely if that small battery fails. They could have just made the Intuvia display battery-free and dependent on the main battery, as most other bikes do. Or, at worst, put in a display battery, but even if it dies, the bike and display should work as long as it's connected to a good bike battery. In product design, that's called "fail gracefully".
Apparently the Intuvia display, with its vulnerable tiny battery, is REQUIRED to have a enough power to send the "Startup" command to the bike system, and to engage the main battery. If your display can't muster enough power on its own, it cannot start the bike.
This whole story reminds me of my Tesla Model S, and my Nissan Leaf, both of which have been turned into expensive paperweights on occasion when the conventional 12v lead-acid battery in them dies after two years. The car computers run off of this old school 12v battery, and if the computer won't turn on, you cannot turn on the car. You are bricked, even though you have a massive and full EV battery. Foiled by the smaller startup battery! Same as my wife's Bosch bike!
Anyhow, if your bike encounters any problems with charging (bad connections, bad battery, main battery disconnected for a long time), your display battery could die. It also will age and degrade over time. There is a risk if you put your bike into winter storage the display battery could discharge and permanently die. Because of the mere existence of this battery, there are now many *new* ways your entire bike system can fail on you. Thanks, Bosch.
So I got out my soldering iron, and replaced the display battery a year ago (like the YouTube video in another comment here). That worked for a few months, but when winter came and my wife rode less, eventually the "Attach to bike"..."See You" messages recurred. And that brings me to today. She just came in the house to tell me she's got the same problem again.
As for the OP, who had replacement parts installed by a shop, and it worked for a few days then stopped. The reason that would happen is they put a new display on, with a working battery, but the recharging is not working for some reason. If that battery doesn't get recharged correctly, after a number of rides with the display on, it runs out of power. Temporary fixes are below.
SOLUTIONS:
Apparently, an easy work-around for a weak display battery, is to simply start the bike from the button on the battery itself. As bad luck would have it, my wife's Raleigh bike has the battery mounted inside the downtube, and has cutout holes for the key and the charger, but NOT the button. So I cannot reach it. (nice design, thanks Raleigh.)
(Update: I drilled a 1/2" hole in the frame, so now she can push the button, and it works well.)
The second work-around is that there is a micro USB charge plug on the side of the Intuvia, and you may be able to put some charge into your display with a phone charger.
Lots of people (not me) seem to have a "Warped mount" problem, where the mount is not integrated well with the bike handlebars, and gets just a bit bent out of shape. This causes bad connections where the Intuvia slides in. Sure, you should try to clean your contacts, and inspect them to make sure they are springy and clean. But also, try loosening the mount on the handlebars to see if your display connects better like that.
