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.