If you want to be exempt from social security, you must fill out form 4361 and send three copies to the IRS. If approved, they will send one copy back marked "approved" and you must keep this in your permanent records. You are then permanently exempt from paying self-employment tax on any earnings for performing clergy/ministerial work for the rest of your life. The choice can not be revoked. However, you must still pay self-employment tax or social security tax for non-ministerial work.
I don't quite get how you could forget that you submitted the form and received the exemption. Certainly, without having that form saved in your permanent records, you could have a problem if the IRS audits you and they lost their copy or claim they never had one and you can't present your copy.
I suppose that if you think you have an approved form 4361 but lost it, you can try calling the IRS general number. 1-800-829-1040.
https://www.irs.gov/uac/about-form-4361