You can choose to check an option in Sim properties that will take your learners back to where they left off after exiting a learn or test mode Sim.

Under the Sim properties -> Learner Settings ensure you have checked "Save Learner's Learn/Test Mode Progress".

From here, if your learners take the Sim in learn or test mode and leave the Sim incomplete (whether it be closing the browser, internet disconnection), when they get back to the Sim it will take them back to where they left off.