My keycap got dislodged. How do I put it back?

I took my keyboard out of my backpack and saw that one key (letter K) had been dislodged. How do I put it back?

Additional info: i’ve had this keyboard for around five years and this is the first time this happened. I used to religiously put it back in its case before putting it in my bag but lately I’ve been rushing and just stuff it in my bavkpck. I’m guessing it got caught in my pencilcase or planner. The keycap and one black plastic square thing was at the bottom of my bag.