What are the requirements to become re-certified as a COR after a few years of not maintaining the continuing education requirement due to being in the private sector?
Based on your e-mail address the Agency you work for is USCG; it is most likely you fall under the FAC-COR program. While you were on active duty you probably fell under the DoD Certificaction and Training requirements. The two are separate and distinct.
See the Federal Acquisition Institute FAC-COR page for guidance on certification and training requirements. Although it addresses "grandfathering of certification"; I could not find where it address the "break in service" scenario or recognizes previous certification from DoD. This is most likely going to be a case-by-case decision from your current chain of command and the contracting office supporting your organization.
For DoD, the training and certification guides, rules, and procedures are found in DoDI 5000.72; see specifically Enclosure 5. Two of the biggest differences are as follows: 1) DoD does not "certify" individuals as CORs, in other words you will not get a certificate saying you are a level I, II, or III COR. Training, experience and certifiaction is done at the contract or order level. Finally DoD uses requirement categories A,B, or C. You could be a COR on Requirement A Task Order and at the same time be a COR on a Requirement C contract. 2) In DoD; if you have not performed COR Responsibilities within the past 24 months... the training requirements clock starts all over again.