What data will I lose if my machine crashes while I am updating an object.

When you check-out an object in Edit mode, the object is released from the database. Until you check it in again, the object will not be stored in the database.

Hence, in case of a crash in the client machine, we can restore the object to the last checked in state.

This is why we always recommend that the Modelers Save their work every now and then. Because, in this scenario, the object will be restored to the last saved state.