I Was Just Sitting At My Desk, And I Realized

I was just sitting at my desk, and I realized I couldn't hear but muffled and what appeared as distant noises. Then I realized that I couldn't see. The text on my computer screen was like trying to read it through several inches of water. I could hear my heart thumping and it was LOUD. I realized I was losing consciousness, so I took several deep breaths and after about 45 seconds or so, things began to resume to normal. I waited 5 minutes, but I was very shaky to stand up and walk. What was this?