| it would help to know what kind of orchid you have as different orchids require different care. Hafing said that, the kind of leaf drop you're talking about can be caused by root loss. Being in a container that does not drain is ideal for causing root loss due to rot.
i suspect that if you pull the plant out fo the pot you will find most if not all of the roots to be mushy and grey/black. If that is the case, cut them off. Leave any that are white/cream/green and firm or crispy.
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