Phal stem turns black! Help!

[brutallica_bg]

hi,
I have a great share of misfortune with orchids this year.
i have a new problem now. I have had this orchid for 5 years and I have changed nothing in the way I look after it. It bloomed every year. And it looked healthy. And it looks healthy now. The leaves are fresh and green, after I repotted it a lot of new leaves grew. the roots seem to be healthy too. about a month ago a stem appeared and started growing. it was about 5cm long and with one very small bud, when out of the blue the bud turned yellow and withered. since the same thing happened to my second orchid this did not bother me much. but things did not stop at that point. the stem stopped growing, started turning yellow on the top and in no time turned black. now the "blackness" creeps down the stem. what do i have to do? is it very dangerous? only the stem seems to be in this condition. the plant itself seems healthy. can you help me?
thank you