You can make a motion to withdraw the plea before the sentence is imposed but not afterwards. Then you have 60 days after sentencing to file an appeal. If both these deadlines have past you may find a lawyer willing to file a writ of habeas corpus for you. Beyond that there are no tomorrows.
There are a number of ways that you can seek to vacate the plea agreement. It is usually very difficult to do, but it is certainly possible depending on the facts of your case, and whether you were represented by incompetent counsel.
Most criminal attorneys offer free consultations. Your question really requires a consultation since there are way too many facts that an attorney would need to offer you any meaningful guidance.
Brian (Tustin criminal defense attorney)