Looks like you accidentally made it invisible! There's a couple ways that could happen. On the Properties bar on the left when you click the object, you can set its Initial Visibility and its Opacity. If the first says "Invisible" or the second says "0", you won't be able to see the object.
You also might've made the layer the object is on invisible. Same deal for layers; click the name of the layer in the Layers tab on the right, and then on the Properties on the left, check its Initial Visibility and Opacity.