What is saved
Every answer you have already given. Assessments save each answer as you make it; interviews save each response as you finish it. A dropped connection does not lose work that is already behind you.
Rejoining
Open the same link again. If the interview is still within its time you rejoin where you were.
The player also tries to reconnect on its own for a while before giving up, so a brief drop often resolves without you doing anything.
The clock
It keeps running. The duration is measured on our server from when you started, so time spent offline is time spent.
That is worth knowing rather than resenting: it is what makes the same interview the same length for everyone, and it is why a stable connection matters more than a fast one.
If you lost a lot of time
Finish anyway. An interview you completed with a gap is something the organisation can look at and judge; an abandoned one leaves them nothing.
Then tell them what happened and roughly when. The session records the disconnection, so a genuine outage is visible rather than being something they have to take on trust.
Preventing it
Wired beats wireless. Wireless beats mobile data. Close anything streaming or syncing in the background — a large upload competing for the same connection is the commonest avoidable cause.