The only things that can play a part are:
Tone map settings, you didn't show us those, the exposure could be way down
Something is blocking the sun, this could be a partially transparent object, or a volume such as fog. In fact, we know the curtains are blocking the sun, so are they set up to allow enough light through? Are the interior lights working normally, what happens if you hide the curtains, that would be how to test if the curtain material is to blame.
The Sun size was changed from the default
Hope this helps!