How should I proceed if my I-526 petition was approved for EB-5 after I failed to file my I-829?

Unless you can get an excuse for late filing the I-829, you need to start over.

USCIS allows for an I-829 to be filed late for good cause. I would first recommend trying to late-file the I-829 (assuming you have a valid reason for not filing or arguing that you do) before giving up on the entire process and filing a new I-526. If your I-829 is not accepted, you will have to reinvest in a new project and start the process anew.

This sounds like a call with an immigration attorney would be best for you, to discuss what was filed and when.

If I understand the question correctly, you can submit a new I-526 based on the previous investment (assuming it has been sustained) but you will need new supporting documents as years have passed since you filed the initial I-526.

If there is a reason to late file the I-829, that would be the best option because the receipt automatically extends your green card status. If that fails, and you have not taken out your investment, and the requisite number of jobs were created, you can try a refile, but petitions are taking over 2 years.