What happens to the validity of a degree obtained from an accredited college if they lose accredidation years later?
Things that a school does or doesn't do after a degree is bestowed can impact the schools reputation and thereby impact the utility of the old degree. So the degree would remain accredited forever but if the school lost its accreditation, it is likely that the utility of the degree would suffer somewhat.
Technically, nothing. Practically, it might be hard for someone to verify the degree presented. This subject has been discussed at great length on this board. A search through the threads would reveal much.
The North Central Association maintains a list of formerly NCA-acredited schools and the years they enjoyed that recognition. This list includes both schools that lost accreditation and continue to operate, and schools that closed or merged with other schools.