Leaves rapidly turning yellow .Please give me some advice.
 

Virtually overnight my orchids leaves are turning yellow:( .
I am not an expert and was wondering if someone can give me some advice on what to do.
Thank you in advance
Zeljka

PS.I have attached some photos ,please have a look.

First of all, welcome to OB zeljka!
It's hard to tell from the pictures, but how many different phals are doing this?
In the fifth photo, if the phal only has that one leaf that's yellow, I don't think it is a problem as it appears to be the olderst leaf. Phals regularily shed their olderst leaves.

Usually, the first thing to do when you notice problems is to check out the roots, as root rot can lead to leaf yellowing, among other things. But if all of them are turning yellow at once it could be other things. Did they get overly cold one day/night?

I agree, check the roots! Good roots will be firm, bad ones squishy! I would also say squishy brown roots are bad, but some potting mediums can turn the roots a tan/brown color, so if they are tan/brown and firm then the roots should be ok. You will need to trim off any bad roots. When was the last time you repotted them? What kind of lighting are they getting?

Welcome to the board and I'm sorry you're having problems with your orchids.

I noticed in some of your pictures there are fat, healthy roots and still the yellow leaves.

I've had problems like this several times and have never found the true answer, even with totally healthy plants and roots.

Picture number six looks like some type of cell damage. Cold water? Cold draft on them? Over watering, under watering? Too much light?

The best thing I can recommend is making a check list of everything pertaining to their culture, starting with a look at their roots and potting medium.

Have you sprayed them with anything? Could they have bugs of any kind? Mites? Too much fertilizer?

Sometimes it takes a lot of questions to find the right answer. Good luck and please post on your findings.