Can’t log in to site B on my computer?

The inability to log in to a specific website on a single computer is almost always a local client-side or configuration issue, not a problem with the website's core service. The diagnostic process should systematically isolate variables, beginning with the most common and easily remedied causes. The primary suspects are typically cached credentials, browser-specific data corruption, or an overly restrictive local security setting. Before any complex troubleshooting, a fundamental step is to attempt a login using a different browser on the same machine, or the same browser in a "private" or "incognito" mode, which loads a fresh session without extensions or cached data. If login succeeds in this isolated environment, the problem is definitively localized to your primary browser's profile. The most effective remedy is then to clear the site's cookies and cached data specifically for Site B, or to perform a broader reset of cached passwords and autofill data within the browser's settings. Concurrently, verify that browser extensions, particularly password managers, ad blockers, or security plugins, are not interfering by temporarily disabling them.

If the issue persists across all browsers on the computer, the scope of potential causes expands to machine-wide network or system configurations. A corrupted local hosts file or an overly aggressive firewall rule could be blocking communication with Site B's authentication servers. More subtly, an incorrect system date and time can invalidate security certificates, causing a silent failure during the login handshake. Network-level issues can often be flushed by clearing the DNS cache via the command prompt (`ipconfig /flushdns` on Windows, `sudo dscacheutil -flushcache` on macOS) and restarting your router. It is also critical to determine if the login failure is consistent or intermittent; an intermittent failure, especially one accompanied by timeout errors, points toward network instability or problems with intermediary proxies.

When cross-browser and basic network checks yield no results, the investigation must consider account-specific factors and deeper software conflicts. Ensure you are using the correct login identifier for Site B, as many users conflate usernames with email addresses. If password recovery emails are not received, check your spam folder and any email filtering rules. On a software level, particularly on Windows, third-party security suites or outdated SSL/TLS libraries can disrupt modern web authentication protocols. In rare cases, a conflict with VPN software, even when not active, can alter network stack behavior. The definitive test to rule out a broader account or service issue is to successfully log in to Site B from a different device on the same network, such as a smartphone. Success from another device confirms the problem is isolated to your computer's software environment, while failure suggests a potential network-level block or an account issue requiring support from Site B's administrators.