Credit Score After One Missed Payment

Last year my husband and I were considering buying a house, so we started looking into loans. Our credit was checked twice in April, and we were told it was great! Over 720. Ultimately, we decided to rent for another year. We opened a new credit card with a $1000 limit this past July. In October, I was rushed to the hospital at 32 weeks pregnant and had an emergency c-section, which has been 100% covered by insurance. With us rushing back and forth from the NICU and trying to help me recover, my husband accidentally paid one of the credit cards late. According to Turbo, his score is now 527 because of that single late payment.
Why???
And how can we raise it? Thanks!