I-130 denied but found out 8 years later

I'm a US citizen and I filed an I-130 form for my brother in 2010 from CA. Up until 2018, the status online would show under review. in 2018, the status changed to "denied" stating a denial letter was sent in 2010 with reason for denial. I was surprised as I received no such letter. I requested the letter several times online through the online form since, but have never received any letters yet. It has been 5 months since the status changed. I'm not in US currently but will be in US in July. What should I do?