Assuming you have a masthead rig, try the following:

take a piece o line long enough to reach the masthead and tie a
slipknot in it with, say, a 12" diameter loop in it. Make sure that
you tie it so that the part to pull to close the loop is on the (very)
long tail end.

Now, tie the short end on to your main halyard. Raise the
halyard/loop combo to the lost piece of your roller furler. Lassoo
the beast, pull the loop tight and then pull the assembly back to the
deck.

Depending on what is at teh masthead, this may be easier of harder
than going aloft.

Good luck!