ISAP is typically a program for people who are in removal proceedings who are not being detained in jail. If you are on ISAP, you should be notified of the dates and times of your immigration court appearances.
The first step in the immigration court process is when ICE gives you a "Notice to Appear." This means you are in removal proceedings, but it is only the beginning of the process. You should contact an attorney to determine if there are ways to stay in the United States.
You probably have an order of removal against you. SImply ICE is not enforcing it, or better was not enforcing it until now. Contact an immigration attorney. You need to look into your convictions and into any motions to reopen. Also you want to see if you may, due to your condition, avoid the enforcement of removal by ICE.