Do you mean that the company isn't displaying on your Gear icon (⚙) > Switch Company screen, or that the company's information isn't appearing when you load it? Assuming there hasn't been any user permission changes preventing you from accessing the company, I'd recommend clearing your browser's temporary cache files.
Web-based platforms like QBO collect cache files (cookies) to save you time when loading repetitive data and images. These files can often become outdated or corrupted, causing issues like the one with your 2nd company. Clearing them out may allow you to properly load the information, and here's how you can do so in all supported browsers:
By doing this, you'll force QBO to load a fresh copy of all the information in the company you're trying to load. Please know you can always find me here for all of your QuickBooks questions. Thanks for coming to the Community, I'll keep an eye out for your response.