When an older mid-large dog suddenly does not want to move much and is struggling to stand, it is typically a musculoskeletal issue.
We would consider e.g. hip dysplasia, advanced arthritis or a slipped disc in the back.
This can occur without an injury, especially if things have been worsening in recent times.
Another consideration would be weakness due to dehydration and/or a toxin ingestion that has worsened any pre-existing joint pain and made walking a lot harder.
The panting may be due to discomfort and stress.
Given all you have said, I would want Jack seen by a vet immediately.
This is so we can ensure all of his nerves are working as they should and so we can assess for pain.
He may well need some medicine such as anti-inflammatories and pain relief.
If dehydrated, he may need a fluid drip and an anti nausea injection.
At this age, dogs cope worse with being unwell and often need medical attention to get them through.
Going forward, we may find he needs some medicine to assist in his mobility.
I hope this is all ok and please let me know if there is anything else,