From the description you have given it sounds like he may have had a seizure. These can happen at any point in a dogs life but when it happens to an older dog a serious illness, toxin, or possibly a brain tumor will have to be suspect. Because events like you described can be attributed to a number of issues it is important to have the dog evaluated by a veterinarian for bloodwork and a good listen of the heart. Things that will need to be ruled out are liver disease, kidney disease, heart disease, toxin, and hypertension. If all tests are performed and clear then the seizure is considered idiopathic or without a known cause.
Please write this episode down and if there is a repeat episode time it. How long does the seizure last and how long it takes him to return back to normal. Be sure to note loss of bowel and bladder control. Time of day and recent stressors are also important.
For the time being I would be sure that the stairs are blocked off and he is protected from any falls or injuries with a repeat episode. You can comfort him when this occurs but be careful not to put your hands around his mouth. Having a blanket or two around will help to protect him from injury by creating padding.
Please let me know if further assistance is needed.