The first thing to do is not to panic.
The next step is to check on the Feedback and Support board to see whether there has been any discussion by other traders, and to post about your problem if you want to.
If for some reason the HSX community boards are not available, you can can check HSX's Twitter account to see if there are any updates. You can also check third-party sources like the HSX Dude's Facebook page and Twitter feed.
It's a good idea to protect yourself from HSX outages. If you're planning to make a large number of trades, make a couple and then check your trade history to make sure they are going through. It can save you from a lot of frustration.
Similarly, you can use Chimp, a downloadable trading widget, that allows you to trade by mousing-over then clicking, rather than keying in trades individually. It makes trading (and therefore re-trading) a lot less work.