Loss of lift.

The wing takes the plane's forward speed, and transforms some of it to Lift and some to Drag. When lift exceeds the plane's weight, the wing will hold the plane off the ground.

As speed reduces, lift reduces, and when the wing's lift no longer exceeds the weight, the plane falls out of the air. Hopefully, you're near ground level, over a landing strip, at the time.

There are other ways to reduce lift. Bank 60 degrees, and lose half your lift. Pitch up steeply, and you lose lots of lift.

I suspect you just let your forward speed fall below "stall speed", that point where lift does not exceed weight.

Good luck,
Dave Olson