This project is due like yesterday.....

The first pic is how it should look. Rendered to screen at 1600x300.

The second was rendered to disk at 3200x300.

Now there's this dark dirt/grain all over everything. And now it's there no matter how I render it.

Global ambience was on in both pics but the grain is there even in standard lighting now. Nothing else is changed. But something did for sure.  Does anyone know what's going on?  Or how to fix it?

If it's been discussed before, sorry.

Thanks,

M