It really is nothing more than a refrigeration unit. As long as the heat exchanger is not rusted through, any A/C service company should be able to get it back up and running. (If the heat exchanger was rusted through,....you would probably have salt water leaking out of it.) If it is not running at all, it is probably an electrical control issue. IE Bad T-Stat.
Reefs are a hobby I have been in for 22 years. A/C refrigeration is something I have been doing for a little over 30 years now. If you was down here in Florida,....I would help you out. I fear up north by the time you pay for a tech to trouble shoot your chiller and then have him order the part you need, reefprobe may be correct. You may want someone to look at it,.....then see if you can contact the manuf. for the part itself. Most of these chillers controls are pretty easy to replace. And the manuf. can walk you through it.