It's likely more annoying than it is urgent. It looks like the hole where the bolt should be is rough and oversized, so you probably would just need to get a big enough washer to be able to secure it back to the car. You could also go to a junkyard and see if you can get a replacement heat shield that doesn't have worn out holes (giggity)

This is a heat shield. It is there to keep the heat of the muffler off the underside of the trunk. It is not really dangerous but if it went to fall off then you might melt any carpet in the trunk. All you need to do is find someone to put a few quick words on it and tack it back into place. It will take them longer to pull the tools out then do the welding.