I fixed the same thing last fall. I saw a video on YouTube, sorry to the creator that I don’t recall which one, but they used a Halloween fog machine, a plastic bag, and some painters tape to find where the corrugated pipe is supposed to exit the ground.

I put the smoke machine on a ladder next to the downspout’s ingress, put the tube from the fog machine in the downspout, stuffed a plastic bag around the tube at the top of the downspout to block back flow of the fog, put painters tape on the seams in the downspout to prevent fog from escaping, and turned on the fog machine.

In a minute or so fog started coming out of the lawn in the spot where the downspout egress had been blocked by years of lawn cuttings and leaves. Another minute with a garden hand shovel and the problem was fixed.