Once you have logged into your Google Webmaster tools account, click on any of your listed / claimed domains to get to that domain's dashboard on Webmaster tools. If you look to the left-hand sidebar of the screen you will see the typical menu (see image below). You will want to click on Crawl to open that drop-down menu and from there select Fetch. Your screen should look something like this (initially yours will be mostly blank):

As you can see from the image above, we have gone through the trouble of already trying to "Fetch as Google" the homepage of this domain... and wouldn't you know it - Google can't fetch it! WHOOOPS! Do you think this might be part of the reason why this domain's search rankings have plummeted? That may or may not be the case... but with results like this, it might not be a bad place to start.