Most trees give a death date of 3 Apr 1788 in Amherst, Virginia, sometimes citing Geneanet Community Trees Index.
A minority of trees give a death date of 7 Oct 1796 in Elberton, Elbert, Georgia.
Neither date is adequately sourced, and no sources are easily found online at Ancestry.com, FamilySearch.org, or AmericanAncestors.org.
If she died in 1788, then her husband remarried, to another Margaret, as he mentions his wife Margaret in his 1792 will.