You shoould take it to your local guitar shop and have the bridge re-adjusted. It is a fairly inexpensive procedure..and if you'd like, they will probably show you how to re-adjust it yourself. The procedure will have to do with adjusting the springs in the back.
You should definitely remember to only take one string off at a time when changing strings. The reason the bridge came up is because it had no strings to balance it.
Ryan Buckner, Ysrafel