There are a few possible scenarios.
If all other users from that company can access it, but that particular user cannot, the company is likely blocking your site for that user.
If none from that company can visit, but you can visit the site from the same internet provider’s internet that the company uses, it is likely blocked by the company for all.
If you cannot access using the ISP’s internet connection, it is likely blocked by the ISP.
Also, check if your domain or server IP is listed in any blacklists.