If the instance hasn't been destroyed you need to get the instance id... https://github.com/rathena/rathena/blob/master/doc/script_commands.txt#L9093
When you have the instance id you can then use the instance_mapname command and warp the play back to that instanced map.
https://github.com/rathena/rathena/blob/master/doc/script_commands.txt#L9085
The full thing might look like...
warp instance_mapname("prontera",instance_id(IM_PARTY)), 0, 0;
This is if the player in question started an instance of the prontera map and we wanted to warp them back there.