Most likely all you've done is spun the stem relative to the front fork which is very easy to fix and generally only requires an allen wrench to fix depending on the vintage and design of your stem.
If your bike is relatively modern you'll have a threadless stem and you adjust it like this: http://www.youtube.com/watch?v=3SuTxugV6yY
if you have an older quill style stem (used with threaded headsets) you just have to loosen the single bolt mounted vertically through the stem, perhaps tap the bolt to loosen the expander, rotate the stem back into position and re-tighten the expander bolt. It looks something like: http://www.youtube.com/watch?v=ddKuYvQXsTs
Good bike maintenance advice here as well: http://www.parktool.com/blog/repair-help/categories/headset-service