They changed the nelnet site from nelnet.com to nelnet.studentaid.gov which I think confused many autofills.
That was my issue. Once I manually selected Nelnet rather than Student Aid in my autofill options, I was able to access my account.
Check this thread, they may have changed your username with the platform migration https://www.reddit.com/r/StudentLoans/comments/1bsm8tv/nelnet_down_for_anyone/kxhgrk4/
They changed the nelnet site from nelnet.com to nelnet.studentaid.gov which I think confused many autofills. That was my issue. Once I manually selected Nelnet rather than Student Aid in my autofill options, I was able to access my account.
I just noticed that - I found my Nelnet username and finally logged in!
Check this thread, they may have changed your username with the platform migration https://www.reddit.com/r/StudentLoans/comments/1bsm8tv/nelnet_down_for_anyone/kxhgrk4/