The possibility is there d going to jail, but it depends on why you didn't do what you said you were going to do. How long ago you were supposed to do it and how many times you failed to complete the requirements. As you can see thre are a of factors involved, it's not black and white, yes or no.
You need to consult with a criminal lawyer, explain your situation, and hire them to assist you in lobbying for why you should not be thrown into jail and why the warrant should be recalled.
Things to consider that might mitigate your situation with the court:
Were you still in-patient when your court date arose? If so, take all your paperwork showing you were in-patient.
Did you before or do you know have the ability to pay the fines and fees? If not, advise the judge and ask to be put on a payment plan which you can manage.
Have you been drug testing regularly as a condition of staying connected with your rehab? If so, take the results with you to court.
Is there any other reasonable reason which prevented you from going to court? If so, tell the court.
The lawyer who rep'd you on Prop 36 should accompany you to court on this warrant. If you would prefer, you can always hire a different attorney to assist you, as my colleagues have suggested. The lawyer will put you back on calendar.
You should get back to court ASAP. The longer you wait, the worse it is -- not only because it will look bad to the judge; but because an open bench warrant means you can be arrested any time (in most cases). You should bring an attorney with you when you turn yourself in, and have them make the best case possible for you in front of the judge (the other attorneys' answers have addressed this in detail).
If you cannot afford an attorney, you should still go ASAP. Go early in the morning so that you're at the criminal court clerks' office by 8:30 am. Explain that you have a warrant for FTA and you want to appear in front of the judge. That will get the ball rolling.
While you're right to be fearful of jail, the situation (and likelihood of jail time) will only get worse the longer you wait.
Retain an attorney (or go back to the attorney who represented you previously) to immediately address the bench warrant. The warrant will be recalled, and you will most likely face a probation violation. Depending on what number violation this is, you will still be able to complete the program. Hopefully this is not the 3rd violation, which means you could get kicked out of prop 36.
Do not delay--these types of matters will produce the best results the sooner you show the court you want to get on course again. If money is an issue, a payment plan can be set in place. Missing court is a bigger issue. An attorney may effectively explain to the judge why you missed court, which hopefully was for a good reason.