You need to give more information than what you think you did not do.
What is your growing medium? If soil, what was the mix or proportions you started with?
What lighting? Indoors? Outdoors?
PH of soil and water if known, along with feeding schedule and types of nutrients used.
Without more information, anything we answer is nothing but a wild assed guess. SInce you appear to be in soil, I figure it may be some sort of over or under nutrient issue.
There is a stickied post that has lists of common problems with links and pictures. Check it out and if it isn't obvious, then let us know the above answers and someone smart will help. (Not me, I am out there flailing myself!
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