Take it back. If you have a bottle of compressed air, you can try spraying that under the space bar. Do not use a vacuum to clean it in anyway. You could also hold the laptop on its side and blow hard into the area under the space bar as well. Depending upon what you brought it in for, they may have removed the keyboard (which is rather easy) and didn't put it back on correctly.
@iamnikki They snap out pretty easily, and sometimes don't get snapped back in correctly. Some laptops you have to remove the whole keyboard to clean it.