We just pick up where we left off and over the years I've learned not to stress about it. Like Dianna said, our learning takes place 365 days a year.
I will say that things can get a little trickier w/ high school kiddos. Last year my dd had surgery, months of PT, then very sick from a med she was placed on it did mean in the end that we covered some school in the summer and some on weekends but it was an extreme case that hopefully we'll never face again w/ our dc.
My dc have had the opportunity to spend parts of several school years with grandparents who were on their final journey. While we tried to cover basics most days during that time -- meaning math, LA, reading and I did read alouds w/ them -- school was time spent w/ family, school was helping and offering assistance to someone in need, school was meeting Dr's and nurses, school was visiting w/ people who stopped by to spend maybe one last visit w/ grandma or grandpa. If my dc had been in public school there is so much busy work that would have needed to be made up -- but because we homeschool life was our lesson and believe me they learned so much more and talk about those experiences more than any textbook they would have read.