I am sorry to hear you are worried about your St. Bernard. My name is Rebecca. I am a veterinarian and will do my best to help.
Falling to the ground and not getting up for awhile could be serious. It could be his heart; it could be a neurologic problem, even a type of seizure; it could be muscle weakness; it could be orthopedic (joint or bone problem). If it happens again, I would immediately look at his gums to see if they are a normal bright pink or if they look pale; if the gums are pale when he is down this might be a heart problem, not enough blood and oxygen circulating.
Weakness can be from electrolyte imbalances (sodium, potassium), muscle disease, arthritis.
So, many things could cause this. Were any blood tests done when he was checked out 6 months ago? Is he on heartworm preventative?
I would start by having him checked out again; a 10 year old St Bernard is an old dog, and a lot could change in 6 months. If blood tests were not done then, I would consider having some basic blood tests done.
Please let me know if this helps, and what other questions I can answer.