Thanks so much both Aaron and silken for your replies.
My mum is checking at the moment (she knows about plants tho not about orchids, but nevertheless), and reckons that you might be right and the sudden change of temperature might have caused this. Is there anything special I can do to treat it or should I just keep with her in a normal way? (In Spain "orchid" is feminine, so don't be surprised if I refer to the ones I have as her.
About your question, silken, this is basically what happened.
I had two orchids, one in June and one in July, both nice, in bloom, healthy and with good aerial roots. But both, just after a week here, got all the aerial ones dried. I kept watering them the way they suggested at the Garden Centre (dunking them during 30 minutes in water and then leaving them to drain) and once a week as it was Summer. I think that was the mistake as the roots started to rot.
A couple of months before Christmas I got rid of all the rotten ones and changed the watering method, once every two weeks and only a bit of water with a bit of growth solution, and placed their plastic pots on top of wet pebbles at the bottom of the "decorative" pots. That way they had a bit of humidity they could hold on to. It seemed to work as they started sprouting really nice and one of them had a bud that was supposed to be open by the time we got back from Christmas.
Instead it went wrong and the bud was all dried when we got back. I think either the fact that there was no one in the house so it was very cold, or that the person who staid had the heating full blast every day, caused a shock in that orchid.
Afterwards one of them lost one of the sprouts, and one spike on each of the orchids (they're twins) dried out and stopped growing. (You'll be able to see it in the pictures.)
They advised me where I bought them to do what I had in mind, repot them and get rid of all the rotten roots they might have had. Also they suggested to try and keep them in a dormant period for 4 weeks so they could get their strength back. As they are not blooming and one spike on each is not growing, they wouldn't need to put too much effort in any of those tasks. In the end one ended up with one healthy root and the other with one and a half.
Funny enough, the one with only one root had a new one growing as I could see the tip.
The temperature in the room is always between 15º C and 19º C. Haven't watered them since I repotted them (a week ago) and only spray them in the mornings so they can have a bit of humidity. The very last picture with the blue pots is how they ended up after my attempt at trimming them.
We suffer from mold and humidity problems in the house, which is the reason why it's so dry in the living room. My first orchid I had was 3 years ago on July, and because it was a small room and I used to dry my clothes there, it had always enough humidity, plenty of indirect sun light and watered enough. My landlady kept it after I left as it was her present and I hated her... (Shame because I loved that orchid.)
Now I'm spraying the new one (the one with the "cold") every morning and some evenings, so it can have enough humidity as I don't want it to lose the aerial roots as well.
I know I might be taking too much care of them, but the difference from one place to another is not making it easy to know how I should act.
(Sorry for the long writing!)