My iTunes library has disappeared, help?

I have a MacBook Pro, which I just recently purchased and am still not very familiar with as I was a PC user prior to owning a MacBook. I have an iPhone 4s, too, but my dilemma doesn't exactly stem from that.

Here's my problem:

When I got this new MacBook, I plugged my iPhone into the USB port, I signed into my Apple ID, and even though most my library contains downloaded music from other sources, I had all of my purchased music, movies and TV shows in my library. I watched my TV shows and movies last night, I swear they were there, but now they've just... disappeared. My entire iTunes library has gone back to the default screen where it gives you the option to scan for media like it did when I first purchased the MacBook, before I had signed in. I am currently still signed in, but it is not displaying any of my purchases - music, movies or TV shows! It will show me my purchases, but just the trailers or the the clips, but my purchases are not actually on my computer. I don't know what I did.

Help me, please?