What happens if you stop taking sertraline abruptly?

Discontinuation. Stopping an ssri abruptly can cause a "discontinuation syndrome" which can be very uncomfortable. People can experience this as physical symptoms, emotional symptoms, or both. It can be especially strong with any of the ssri's that have a short half-life (prozac / fluoxetine has the longest). Some physical symptoms are dizziness, especially with eye movements, electrical sensations in head, etc.