They may have a sour taste in their mouths following the DNC hacks that revealed Bernie Sanders' chances at the nomination were sabotaged by his own party, but ultimately Sanders supporters have no choice but to vote for Hillary Clinton in the upcoming election. By staying home they are effectively casting their votes for Donald Trump, which is something no self-respecting Democrat wants to do.
It is unlikely they will vote for Clinton. Many people have already expressed a lack of support for Clinton if Bernie were to not get the nomination. It is not certaint who they will vote for, some possibly for Jill Stien, some maybe for no one, and a few may vote for Trump.
Many Bernie Sanders supporters will not vote for Clinton in the election. Hundreds of Sanders delegates walked out of the convention after Clinton was nominated. Sanders supporters believe that the Democratic National Committee helped rig the primary elections so that Clinton would be the party's nominee. Furthermore, the recent hacked DNC emails show that there was collusion between party officials and the Clinton campaign. All of this will likely make many Sanders supporters not vote for Clinton.
Some Bernie Sanders supporters will march in line with Sanders' instructions to help elect Hillary Clinton. However, those who were angry enough to walk out of the convention will probably not vote for Clinton. They will either stay home on Election Day in protest, or they will vote for Jill Stein.