This happens to everybody once in a while. Unfortunately, by the time a cake has cooled, its leavening has been deactivated and the air holes that create the cake’s light texture have closed and stuck together—so putting the cake back in the oven will not save it.
Check carefully, though—many cakes sink in the center, which may not mean that the cake is underbaked. If you have tested the cake’s doneness with a toothpick that came out with just a few moist crumbs attached, chances are you have just baked a nice moist cake, and all you have to do is disguise the low spot with frosting or whipped cream.
If you find that the cake is runny and raw in the middle, just scoop out that part and serve the rest.