Phalaenopsis leaves, yellow, black at base

[camille1585]

The stem turning brown after flowering is not a worry (perfectly normal), but the black mushy rot where the leaves fell off is bad news. The fact that it's not the lowest leaf that fell off first, and that you lost 3 leaves in quick succession also seems to indicate that something is going on.
One thing you can try is pouring some undiluted 3% hydrogen peroxide over the infected parts (I use an eye dropper to do it) and for good measure pour some at the base of the remaining leaves as well. H202 is a strong oxidizer that can be very effective against bacterial and fungal infections.

Unfortunately if the infection has already spread to the inside of the main 'stem' of the phal then it's likely a goner.