Do you have a photo ? The "brown spots" on ferns can often be spores....the fern equivilent of seeds. Give them a rub and see if they are like a fine, brown dust that come off on your fingers when rubbed.

If not, i would possibly imagine that your fern might of dried out or is in too much sun. Need a bit more info.

If you live in an area that has cold winters then this could it be cold damage to last year's foliage?

Thanks

If the edges of the leaves are turning dead and brown, it might be lack of humidity, especially if they are growing indoors. Other common causes are missing a watering or two, too little light, or too much sodium in the water.