Fixing metal siding that is loose on garage
yes. screw it back into place. Use truss-head type screws rated for exterior use. Truss screws have a round very flat head. Drill pilot holes to fasten to frame beneath. Start your repair in the center of the loosened panel; working outwards in each direction. This will decrease the chance of the panel becoming distorted.  Apply 100% silicone along underside of panel to improve adhesion and waterproofing. I'd use as few screws as possible so as to not twist or wrinkle the metal siding. If you use urethane caulking it will not only stop water penetration it will adhere to the metal and framing much like a construction adhesive would. 2 for 1 as it were.