What are the side effects of not taking this medication after taking it for many years?

I imagine you are referring to Adderall, as I did answer your previous post, where you mention Adderall.
With regards to this question the name of the medication is not provided..

What are the most common symptoms of Adderall withdrawal?
These are the most common symptoms reported during an Adderall withdrawal, although not everybody will experience ALL these symptoms.

Anxiety
Appetite changes (feeling hungry or not hungry at all)
Cravings for Adderall or other drugs
Depression
Difficulty concentrating
Headache
Irritability
Lacking in motivation
Suicidal thoughts
Mood swings
Muscle aches
Seizures
Sleeplessness
Tiredness and fatigue
Tremors.

Source: Drugs.com
https://www.drugs.com/medical-answers/long-adderall-withdrawal-3572393/