Since Petco is still having their $1 per gallon sale (it might be over this weekend) the best thing would be for you to buy a new 55 gallon tank for $55. That would be the easiest thing, and if I were in your position it would make me feel better, knowing I have a new sturdy tank. If the brace broke it might be an old tank and the silicone might be old on it too, those seals don't last forever. If the tank was ever stored outside, this will make both the silicone and the plastic trim brittle over time.
The next thing you can do is order replacement trim, if you know who manufactured it (Perfecto? All Glass?). Your LFS should be able to help you with this.
Gluing it back together isn't going to work, unfortunately. The center brace in a 55 gallon is very important, the old style 4' tanks often didn't have a brace because the glass was thick and real glass (not tempered) but with these newer tanks, it is a must to have the bracing. If you haven't already, lower the water level a few inches so there is less water pressure on the top.
Other kinds of repair, like euro bracing, are more trouble than they are worth for such a common tank and require skill and precision, not to mention custom cut pieces of glass.