I sent it back to my vendor, and they replaced the battery. Now, it's OK.
The thing I'm worried about is that it may discharge completely again, and the battery may go flat again. When it happened the first time, I actually didn't know what caused it to drain. I had it charging before I went to bed so I was pretty sure that it was fully charged when I disconnected it from my laptop. Then when I woke up in the morning, it was completely drained. I'm aware that an apps may have caused it to wake up and not go to sleep, but to completely drain in just a few hours?? :shock: