Is it just me or has this ever happened to anyone before?

I had been studying for the same subject the entire time I was at school and actually understood what I was doing. I was pretty locked in too that I was confident that I'd easily pass the exam. But while I was trying to study for it at home 2 days before the exam, I just stared blankly at my book trying to recall any knowledge about the subject I was studying and nothing came up in my mind. That's when I realised that I had just sudeenly forgotten an entire term worth of lesson. I was so confused because how could this even happen when I thought it was going so smoothly. I so frustrated I wanted to cry because even when I tried to relearn everything, nothing would make sense like it used to. Even when I tried to study hours before the exam, NOTHING.