If by 'stem' you mean the inflorescence - that turning yellow is normal after blooming has finished. If you mean the pseudobulbs or canes, no, that's not normal - nor is losing leaves usually. Do you know what kind of orchid you have?
Pix are definitely helpful - I think you need to post a couple of times before you can post pix.
The plant is "Phalaenopsis".
It is the stalk that provides blooms. It bloomed great, lasted a long time, but now as the leaves are falling off, the stems are turning yellow.
Did all the leaves fall off or just a couple bottom ones? After blooming it is normal for the flower spike to turn yellow. But dropping all leaves is not. Sometimes the plant will shed a bottom leaf or two and that can be normal if the roots and the rest of the plant are healthy. Post a pic when you can. It will help us know if something is wrong and how to fix it.
Last edited by Wynn Dee13; 08-24-2012 at 02:03 AM..
Is your phalaenopsis growing in a clear plastic pot? If so, are you able to look at the roots? If there are problems with the roots, then it will affect the rest of the plant. If you can't see the roots, maybe with some pics we on the forum may be able to figure something out without having to disturb the roots as much as possible.