Check that the Garmin now is recognised by your PC and goes into Mass Storage mode. If so, you've fixed the problem!

Clean off all the residual double sided tape from both the screen (very carefully, so you don't remove the black printing on the underside) and the case

Apply some new double sided tape to the inside rim of the case (easier than trying to put in onto the screen I found) I cut some thin strips from some 3M stuff I had lying around that is used for sticking automotive boldy mouldings onto vehicles - doesn't absorb moisture and sticks well enough

Press the screen back into place

Check that everything is working again