We recently had an extensive discussion of how to deal with such an issue, should it arise, in a postdoctoral seminar I attended. A number of senior faculty contributed their ideas regarding how to deal with such an incident.
I would recommend a more gradual escalation than others have suggested. For example, I would not copy the editor of the journal or the head of the department on your initial communication requesting more information. I would start with communication just within your author group: for example, an email to the corresponding author asking for an explanation of why they removed your (and others) authorship without notification, CCing all other authors of the paper. This gives you a paper trail but also allows you to first address the issue without airing everyone's dirty laundry.
If the corresponding author's response is not to the removed authors' satisfaction, you should then, together, think about escalating your complaint (for example, by contacting the editor). I agree with avid that while you would be in your rights to do so, there is a very real possibility of a negative backlash. I think as a consequence the other dropped authors should be on board with such a decision, and you should have excellent documentation of your complaint.
Finally, this type of incident is a very good example of why every paper should start with an authorship meeting where everyone sits down, agrees on authorship and order, and what constitutes the responsibilities of authorship at each level (e.g., "first author will be so-and-so, and that means her responsibilities are to do x,y,z...and if she finds herself unable to meet all of those responsibilities, then authorship decisions will need to be revisited", etc.) Concretizing expectations prior to the work tends to lead to better working relationships and can prevent some very bitter arguments.