We have bought this peace lily plant one or 2 months ago, but suddenly all life seems to have gone out of it. Can we still save it?
We watered it once a week, so I don't think that is the issue.
This is a spathiphyllum, a close relative of the anthurium. The identification key is the white flower with the spadix that drops pollen when mature.
These plants do well in a moist soil and a range of light conditions from medium to bright indirect light. I have seen them very happy sitting in an inch or two of water. When they get too dry they collapse as seen in the picture. To make it confusing they will also collapse if they have root rot but seeing as the leaves still have some turgor I don't think this is the case here.
For this plant, if the soil is not wet, water it. If the soil is already wet let 24 hours pass. If it's still passed out then take it out of the pot and look at the roots. Healthy roots are firm and white or brown. Roots that have rotted are soft and black.
If the plant has root rot throw it out. They are difficult to save at this stage. If it's just dry it will recover quickly.
An aid for these plants is to add a wick and reservoir. It can be as simple as taking a knitting needle to push a wick up into the root ball from the bottom of the pot and sitting the pot on top of some small stones inside another pot. Keep the water level at a few inches so the plant is above water but the wick brings water to the roots and the plant will never collapse again.
Edit: the other ways to tell an anthurium from a spathiphyllum or Peace Lily are that the spath has larger leaves. In some varieties they are six inches long. Anthurium leaves tend to be thicker and a darker green. The anthurium spadix sticks out from the bract known as a spathe whereas in Spathipyllum's the spadix starts enclosed by the spathe and and in many cultivars remains cupped by it.
Peace Lilies can be very melodramatic when it comes to a watering schedule. Even when the plant is drooping all over the place, it can often be saved.
The drooping leaves indicates that it's not getting enough water. This can be caused by three things:
If the soil is not moist, then water until water comes out of the bottom of the pot, and let sit for 24 hours.
Roots actually require air in order to absorb water. If your pot does not have a hole at the bottom for water to drain out of, then the water will pool around the roots, and as strange as it sounds, drown them. The best pots, in my opinion, are the ones that have a hole at the bottom, and a small saucer to catch over flowing water. This keeps your floor clean, and your soil healthy.
Before you repot, you can take a skewer, hard plastic drinking straw, or pen, and just stab them deep into the soil to aerate it a bit. This'll temporarily revive a plant that isn't getting enough oxygen in the soil, and allow you to analyze the conditions further. For example, if it seems to be more root than soil, your plant is rootbound. If there are puddle of water, it's drowning. If there is soggy, brown, stringy bits, then it probably has root rot.
If your plant has root rot, then when you remove it from the pot, and gently soak the roots, many of them will be brown, wilted, and dying. Generally, if a plant has root rot, it's going to be more difficult to save than it is to get a new one. However, I know that peace lilies are common gifts at funerals, and if this is the case it may not be replaceable. You can cut off all of the brown rotting materials, or anything fuzzy, soak the roots in warm water for a few minutes, and repot the plant. Root rot is commonly caused by overwatering, or standing water.
If your plant is rootbound, then you'll see more root than soil, and big thick white/tan roots. If it's rootbound, it just needs a bigger pot, and to have the outer roots broken. Typically, you just kindof rub the outside of the pot shaped soil, and poke it with your fingers, until it's no longer pot shaped.
Here's two different guides on repotting Peace Lillies. http://homeguides.sfgate.com/repot-huge-peace-lily-68569.html http://www.ehow.com/how_2330181_repot-peace-lily.html