My admission is revoked after accepting the admission offer
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.