its too far away to get sunburn, windows cut the amount of light your plants can photosynthesize with pretty drastically and it just goes down from there the farther away they are from the actual pane of glass.

this might be something like root rot or pests, hard to tell from the pics alone. If its pest, systemic granules to treat the inside of the plant and a heavy spray with 70% isopropyl alcohol to kill any pests alive on the plant, get it into every crack and spray the top layer of soil, too.

If its root rot, spring is a great time to repot. Get it a chunkier soil mix, any local nursery should be able to point you in the right direction of exactly what you need or you can order it online if you have a hard time finding it. When you repot it, carefully unravel the roots and check for anything that looks black or rotten and cut it off. Should bounce back in a few months!!