The parties have rules that dictate what happen when a candidate has to drop out. They would likely choose a replacement candidate since there isn't time for another primary. My guess is that they would choose Joe Biden.

Since the public votes for electors and the electoral college votes for president, I think the party could tell their electors to vote for the replacement candidate.

Edit: See this comment concerning the DNC rules for replacing a candidate. A lot of people are saying that Bernie should be the replacement candidate. He very well could be. My reasoning for Biden is that the DNC would want to run a more moderate candidate. I'm not saying what the DNC should do, just what I think they will do.

Edit 2: /u/larry_darrell_ pointed out that Bernie left the Democratic party. That would also make it more difficult to give him the nomination. Also, I don't know about what happens with the ballots since they are set at the state level and the deadlines have passed. I think that's where the electors come into play. They will do what the party tells them to do. Just speculation.