What happens if you completely stop taking Lexapro overnight?

I wouldn't do that if I were you. You need to wean yourself off of Lexapro slowly and even then you are going to get some withdrawl symptoms, including dizziness and nausea. That's what happened to me when I went off Lexapro over a two week period. From what I understand the withdrawl can sometimes be worse than what I had. I would consult with your doctor and come up with a plan for you to get off. If you need to go back on something I would stay away from Lexapro - I have been on several antidepressants and that was the worse one to stop!