It's been a while since I accidentally fired a floppy disk at my sister's laptop, only to discover that it had knocked a key off the keyboard
Although I'm not an expert on laptop button insertion
... you'll just need to try and clip it back on, trying different angles and starting with different clips as the pressure/starting point. Just look closely at the retention clips on the keyboard, and inspect how the clips on the button can clip in place.
The key I had to try inserting back into place had three clips. One hooked and swung on one metal retention clip, and the other two were opposite, and stretched down to clip onto a bottom metal retention clip.