Here are two suggestions. First, it sounds like you actually completed the Breach QUEST. There is no mission for the Breach, but there is a warrior quest. Once you enter the Breach, if you go off to the right you'll hit a bridge with a bunch of Charr (including a Charr boss). That bridge goes nowhere, but the boss is there for a Ranger quest. To get to Piekn Square, you'll need to go left once you get into the breach, down some stairs and into the tar river. Once you're in the river go right (west) and follow it around to a large open area. Go up the left side and you'll find Piken Square soon enough.

As to the main missions, they are only available in areas that say "Start Mission" or "Enter Mission" in your Party Window. The first one is the Great Northern Wall. Here is a thread to the first missions (this will get you to Lion's Arch).

Alternatively, you can run from the Breach to Lion's Arch and beyond (this will take longer, but it's another option). Your best bet for that is to take a look at the Interactive Map provided by this website, because that covers a lot of ground, and the map is the best way to see where you need to go.

Vai