While previously exploring the wonderful world of Skyrim, I stumbled across Korvanjund and cleared the imperials outside and entered to find some sleeping bags and a chest. Now when I try to do the jagged crown quest I can't progress through the mission as the entrance is blocked by rubble. Is there a way to fix this or do I have to start from level 1 again?

Do you actually have the quest accepted? The wiki says the entrance is blocked if you attempt to enter the ruin prior to starting the quest.

Yeah I've talked to galmar and have it as my active quest

Have you met up with Galmar outside the entrance?

Have no idea if this is necessary, just spitballing ideas.

Yeah I have done but when I go through the door there's just a pile of rubble with Galmar on the other side