These vary, but typically the round disc at the ring is a spring or clip fit (like auto dust caps used to be), and can be popped off, and slid up the crank. That should allow you enough access to put a finger in and put the chain back on the sprocket. Also it allows access to the chain case mounting bracket.

As I said, they're not all the same, so don't destroy it struggling.

I usually mount the chains on these by holding the bike so the chain path is sort of vertical and allow some slack. Then I rotate the crank slowly by tilting until the chain catches and I can lead it on. This can be done with the rear wheel on and pushed forward for slack, or using a finger as a rear sprocket or pulley.

I suggest you try my method a while, before going back to the chain case, but if you want to explore so you know for the future go ahead --- carefully.