I was recently experiencing the same issue. I had an old API key that wasn’t being shown on the site, but the API was clearly still working.
I tried going to the Organization API Keys and Project API keys, and neither screen listed any API keys.
What worked for me is clicking “Your profile” on the lefthand menu, under Settings. There should be a tab called “User API Keys (Legacy)” which finally showed my API key. The URL for that page is https://platform.openai.com/settings/profile/api-keys. The page also recommends migrating to a project based API key.
Hope that helps.