An independent clause is a complete thought — it can stand alone as a sentence. Two of them need a real joint between them.
Jam two together with nothing and you get a run-on; join them with only a comma and you get a comma splice. A comma is too weak to hold two whole sentences.
Three legal repairs: a semicolon, a comma plus a coordinating conjunction (and, but, so…), or subordinate one clause with a word like while or although — which turns it into background.
Subordination is a choice of emphasis: the main clause carries the weight. Subordinate the other idea instead and the very same two facts change what they stress. Punctuation isn't decoration — it's the wiring.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.