Firstly, you print out the sample chapters again and send them to another agency/ publishing house, as soon as you can. Rejections are natural: even J K Rowling was turned down several times, and when Bloomsbury accepted the first Potter book it was because the editor's daughter insisted, or so the story goes. You just have to try again.
Secondly, you write some more. That's obvious really, but it does need saying, because for me at least writing is a daily thing. If I don't put down some words one day then I likely won't the next, or the third, and suddenly the script lying on one side of my desk seems to be giving me disapproving looks and it's harder than ever to start back in. It doesn't even matter if I read back the work I've done, decide it's rubbish (that rejection letter distracted me, curse it) and delete the lot: I still did it, and my head's still inside the story.
Which is why I write, in the end. Someone said to me after I published The Risen King that it's a tremendous thing really, writing a novel, and I suppose it is. But to me it's just part of how I live, what I do in my days and evenings. Send me a hundred rejection letters, a thousand, and it still will be.