If you have powdered sugar, you can make your own frosting.
For white frosting, put some powdered sugar in a bowl, add an egg white, a dash of vanilla, then a little water at a time until you get the "frosting" to the consistency you want (either thick or runny or in between). Taste to make sure you like the taste (need to add a little more vanilla? Maybe more powdered sugar?)
For chocolate frosting, the recipe is just as simple. Put some powdered sugar in a bowl, add an entire egg (need the egg yolk to help turn it dark), a dash of vanilla, then cocoa powder little by little until you get the "taste" you want, and water bit by bit until you get the frosting texture you want. Too much cocoa, and you'll get a more bitter chocolate flavor.
This is the "old-fashioned" way my mom used to make frosting. I actually like this type of frosting the best. If you need precise measurements, check the Internet for recipes for homemade frosting.