First, remove the top cap by loosening the bolt on the top of the stem. Make sure there are a couple millimeters of space between the top of the steerer tube and the upper edge of the top spacer. (Edit: If you kept the same amount of spacers and you had no problem before, this is probably not going to be the problem now, but it is worth checking.)

A common mistake is not pre-loading the top cap bolt before tightening the steer tube clamp bolts on the stem. Loosen the bolts that clamp the stem around the steer tube (enough so you can just rotate the stem around). Tighten the top cap bolt--you want it just tight enough that if you hold the front brake lever and rock the bike forward and back there is no play, but not so tight that the fork/wheel can't rotate left and right easily. Align the stem. Re-tighten the steerer clamp bolts on the stem.

Also re-check the bolts on the front of the stem that clamp the handlebars.