Did you bare root the plant when you repotted? Did the roots look rotted or smell sour? If they did, you have more work to do.

The plant is probably salvable, but it will take your being able to provide it with more light and getting your watering habits molded into something that ensures neither under or over-watering. A wooden skewer or chopstick stuck deep into the soil mass is a good way to gauge whether it's appropriate to water or not.

The soil looks very water-retentive, which will work against you over the longer term, but can be worked around for the short term with careful attention to your watering - until you get your plant back on track, at which time you might wish to consider a soil that makes things easier on you AND the plant.

Answering the questions in the opening paragraph would allow us more insight .....

Al

It's still alive as long as the stalks are green and flexible, so there is still hope, even though it's very close to dying.
Does the pot it's in now have drain holes? It is important that the water can drain out well. Peace lilies are kind of fussy about watering. They don't like to dry out for long periods, but they also will rot if they stay soggy all the time.
I would suggest checking it by putting a finger in the soil every day and water it when you notice that the soil is mostly dry. That way you can water it before it starts to droop but still give it time to "breathe" by allowing it to mostly dry out.
Can you open the blinds so it can get all the light from that window? Most likely it is not getting enough light right now with the blinds down. Knowing what direction the window faces would help in determining how close to the window it should be.

@Al: I'm not sure what "bare root" means (I don't know much about plants at all), but I did get rid of all the old soil. The roots didn't look rotten and I don't recall a smell...

I bought Miracle Gro Moisture Control potting mix because that's what the guy at HD suggested. What soil would be better?

@summersunshine: The pot does have drain holes in the bottom. I most definitely can open the blinds; the reason I didn't is because I read that peace lilies didn't like being in the sun. My best guess would be that the window faces west, because the sun seems to shine the brightest earlier in the day.

Thanks for your responses thus far!

Personally, and others may disagree or adjust, I would buy a smaller pot, buy some perlite (google if you don't know what it is), take the plant out, mix up the soil with the perlite so it is AT LEAST 50/50 (I would have more perlite than that to be honest...this keeps it from staying soaking wet and keeps the roots happier), replant, water after two days so the roots heal and then from now on give it as much light as possible. Some direct light in the morning or late afternoon is ok. Water when it starts to droop, which will end up about once a week maybe a tad more in summer.

Normally, repotting again right after repotting is highly NOT recommended but at this point you need to take the risk.