The fine print on the site states, "tropicair.com expired on 03/16/2010 and is pending renewal or deletion".
I sometimes see this message on other sites. When it happened to the bus company (NTSL), it took about 3 weeks to get the site back on line.
Here's the contact information for Tropic:
Phone: 226-2012, 226-3277, 223-0975
In the meantime, try....
Maya Island Air, http://www.mayaregional.com/