ok, some basics: what kind of plant, what kind of medium, what kind of light and water?
also, is this happening to more than one leaf, and what color is the leaf?
the big hairy panic-right-now problem is rot, which will turn a leaf yellow and limp in literally 24-48 hours. it can then eat the rest of the plant on the same schedule.
if you've just got a bottom leaf that is going limp, it may be a watering problem (too much, or too little. helpful aren't i?
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tell us some more and maybe we can diagnose it for you.