Why did you close the top? Sealed terrs rarely last more than a few months to a year at best. Simply too many issues -- the most significant of which is the moist stagnant air situation that results. Such conditions create a perfect breeding ground for many types of molds and bacteria....many of which will be detrimental to the plants or will create an unattractive "scum" on the inside of the glass. Additionally, there is the difficulty of providing enough light for the plants without cooking them. (Think how quickly a car with the windows rolled up heats up while sitting in the sun.)

If your fern is still alive, stick your hand in there and see how wet the media is. (Not just at the surface, go down about an inch or so.). If the media is lightly/barely moist it's okay. If it feels wet, then you definitely overwatered. Overwatering is probably the most common disastrous mistakes people make. If the media.

You very possibly overwatered and overheated/baked your plant.

If the media is wet, your only recourse is to remove the plants and media. Drain/pour out any sitting water in your terr. Take a small handful of the media and squeeze. If water runs out between your fingers it is way too wet. Squeeze what water you can out of the media then spread it out on a flat surface to dry for an hour or so. When it is only lightly moist you can put it back in.

DO NOT use any of the bagged media with "Moisture Control Crystals" or such. Also avoid media with fertilizer already added. Plan on adding fertilizer free perlite to whatever media you use to keep the media lighter/more airy..

I am also having some terrarium woes. I have an open container with a fern that keeps turning brown and I'm afraid that it might die off entirely. I mist every day and don't water. The weird thing is that I have some baby african hostas in there and they love the environment. They've been sprouting new leaves really quickly. I'm not sure what I'm doing wrong. Any advice would be greatly appreciated! I have attached a picture of the terrarium in its location as reference. I have just misted, hence the water. It usually does not have condensation on the sides and the soil is just moist.

How moist is the media?

Hard to tell but the fern looks like it might be some sort of Adiantum (maidenhair fern). If it is, they typically prefer very high humidity. I don't bother with them, myself, as the ones I like grow too tall if they are happy and are difficult for me to make happy to begin with.

Hi Paul, it is a maidenhair fern, yes. It has since died so I've just planted more baby African Hostas in there. They seem quite happy. Perhaps ferns are too advanced for me to handle! Our house is quite dry so I'll stick to the hardier species in future!