Hi Neil,
Thank you for asking for my help with your African Grey. If this is the first and only time this has ever happened, it could possibly have been just a muscle cramp, but the fact that he didn't get right back up on his own concerns me very very much.
It is very possible your Grey had a mild seizure -- Greys tend to be a bit prone to seizures and can have epilepsy but usually it shows before 50 but it can appear at any time. He could also have had a stroke. There could be a number of other problem causing the symptom including organ failure, congestive heart failure, and too many others to name. It is impossible to give you even a really good guess online since almost every one of these possible problems requires at least some testing to determine the exact cause.
He should have the calcium levels in his blood checked as well as other basic blood work performed to check his liver function. Since he is older, he deserves a good look over by his avian vet to ensure he has not had a stroke or possibly even having heart problems.
While I'd love to give you a simple 1, 2, 3 answer, it is not possible to do so with a parrot of his age showing these symptoms. I urge you to take him to a good avian vet immediately and have him checked out.
I trust that at his age, you already have a relationship with an avian vet in your area, but in case you do not, please go to http://www.aav.org and locate one near you. Call and let them know that you'd like an emergency work-in appointment.
While waiting to take him to the vet, keep him warm and calm. I'm sure you love this parrot very much and care for him very well. Sadly, many owners never learn enough to allow their Greys to reach 50, which is really just middle-age for a well cared for healthy parrot. But it is also the age when, just as with middle-aged people, health problems can begin to appear. So, I urge you to get him to the vet without delay.
If I can help further, please use "reply" after accepting this response. I hope your Grey recovers fully and lives another 50 or more happy, healthy years!
Breeder
over 25 years breeding, taming, training parrots.