Elderly cockatiel fell off his perch

Hi everyone, I am new to this thread. I have an elderly cockatiel (20) and he has occasionally fallen off his perch in his cage but has recovered quick and climbed back up to his perch. Today he fell off his perch and landed on his back, wings slightly spread out and feet in the air. He struggled to get back up and was like a tortoise when it gets stuck on its shell. Should he have been able to turn over easily and climb back up to his perch? I left him for about 30 seconds to see if he could manage but he couldn’t so I picked him up and put him on his perch myself. Could it be that he was stunned or is it a sign that something is wrong? (He was definitely stunned bless him). Thanks