I suspect - guess - the maps (or whatever it was you loaded, presumably at home) are held on the SD card. As soon as you take the card out of the device, whatever is on the card will I assume, vanish with it.
Put the new card into your device and follow the instructions. If you want to check how to install maps to your device, Google: How do I install maps to my Garmin Navigator V?
Once Garmin has done its thing. Check that the maps are installed correctly and activated. How to do that is in this thread or ask Google: How do I check which maps are installed and active on my Garmin Navigator V?
To check again, go to say London on the Nav V’s screen and zoom in? Are all the streets there? Do the same in say, Helsinki, Rome, Paris, Berlin and Lisbon, along with any towns and villages inbetween. All there? Looking good, Houston. Then try asking the device to take you from where you are standing, to say 50 miles away. Does it work? Yes? Looking good, still. Then try to load and run one of your (possibly) rediculously long routes….. if it works, job done.
If it doesn’t work, you’ll have to amend the route(s) and find a way to send it / them to your device. Or, amend the route on the device itself.
You might well be wise to do all this, while you are at the hotel and able to use the bikerfriendly hotelier’s PC. The good news is that you apparently created your X thousand mile long routes(s) in MyRoute, which you should be able to access from the hotelier’s PC. Had you created them in BaseCamp or MapSource, you might well be stuffed, unless you saved a copy somewhere accessible.
Good luck!
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Have fun!
Let us know how yiu get on, please.
PS I’d buy the hotelier a decent bottle of wine, before I left. Or, if not possible, I’d send him one from home on my return. He might well have saved your hide.