My admission is revoked after accepting the admission offer I applied to PhD program and gladly received official admission offer on the 29th of March. They guaranteed me four-year funding. I accepted the admission offer and was preparing for obtaining student visa.
I accepted the offer on the 13th of April, declining two admission offers from other institutions.
However, my application page suddenly changed that my admission is rejected. A staff of the department said that they decided to rescind my admission offer on April the 22nd.  What can I do now?
I received this message this morning:

My sincere apology for this unfortunate admission circumstance. There was a misunderstanding after the initial admission process, and the initial determination of &quot;Deny&quot; for your application made by our faculty was indeed the decision that should have been maintained. Later, due to a misunderstanding, we reversed the admission decision from &quot;Deny&quot; to &quot;Offer&quot;. After this change was made, the faculty who had considered your application - and who denied your admission - confirmed their denial of admission due to lack of funding available to support you. For this reason, we needed to change your offer of admission back to the original &quot;Deny&quot; status. Yours was a very competitive application, and I am sorry to have to rescind our offer of admission. We therefore ask that you disregard our offer of admission and funding, which were made in error.
My sincere apology for this misunderstanding, and best wishes to you as you pursue other graduate program offers,
I would recommend the following:


Contact the other universities and ask if they might still be
willing to admit you (this might be the easiest way out). If the
answer is negative make sure to keep their answers for the later
points.
Write to the university that revoked the offer stating (in a
friendly tone) that you turned down other offers due to their offer
and that this situation leaves you (and them) is a suboptimal
situation. If the result of point 1 was negative then add this
information too. Ask them for a solution. If the mistake was made
just on your application then they might consider admitting you to
avoid a law case. If they wrongly offered to all applicants (e.g.
software bug) then they will not be able to do this.
If point 1 and 2 do not lead to anything then write to them a second
time threatening with a law case in order to cover your damages
caused by their mistake. But still in friendly words along the lines that you would prefer not to etc ... (Many people in the comments below suggest to skip this step and go for the lawyer right away "Never threaten to sue. Either do it, or don't"). 
Get a lawyer to discuss further actions.


Other points to consider:


How long was their offer there? If it was just there for 30 min it
might be different to being there for days and they decided to revoke
weeks after you accepted.
Was the offer conditional on anything?
Assume you win with the help of a lawyer - do you want to spend the
next 4-5 years of your life in this environment? I am not saying yes or no - it is just something to consider.