Handlebar alignment with wheel The alignment has drifted such that when my front wheel is looking straight ahead, my handlebar is not perpendicular to the wheel.

How do I fix this at home?
Nowadays most bikes have a 'threadless' headset. The steerer tube of the fork protrudes out of the frame's head tube and the stem clamps onto it. The stem serves to hold the whole system together and apply a little pre-load force onto the bearings to remove any play or movement. Without this pre-load the bearings will become damaged as the fork rocks in the frame slightly as you go over bumps.

If you have a threadless headset and your stem has rotated on the steerer tube, the chances are the clamp bolts are too loose and you have lost the bearing pre-load.

It's not too difficult to re-adjust the stem (and re-align the stem and wheel at the same time).  There are many video's on YouTube that show you how to do it. Here's a reasonably good one.  

If you don't feel happy doing this adjustment a local bicycle repair shop will do it in under 10 minutes. 

If you have an old style threaded headset, the stem is simply secured by a bolt in the top of it. Here is a good tutorial on how to adjust it. The process is fairly fool-proof.