DNS_PROBE_FINISHED_NXDOMAIN fix that actually works

Quick answer for advanced users

Open a terminal or command prompt as admin and run: ipconfig /flushdns (Windows) or sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder (macOS). If that doesn't work, set your DNS to 8.8.8.8 and 8.8.4.4, then restart your browser.

Why this happens

I've seen this error dozens of times, usually when a client's router gets a bad DNS response from their ISP. Last month, a small law firm had their whole office unable to access their client portal—turns out the ISP's DNS was redirecting to a parked page. The error DNS_PROBE_FINISHED_NXDOMAIN literally means your DNS lookup returned "no such domain." It's not a connection issue (your network is fine) and not a router problem (the packet gets there). It's the DNS server saying "I don't know that address." Usually, it's because your DNS cache is stale, or your ISP's DNS is having a bad day. Sometimes it's a browser cache issue too.

Fix steps

Flush your DNS cache. On Windows, hit Win+R, type cmd, right-click and run as admin, then type ipconfig /flushdns. On macOS, open Terminal and run sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder. On Linux, run sudo systemd-resolve --flush-caches (or sudo resolvectl flush-caches on newer systems). Restart your browser and test.
Change your DNS server. If flushing didn't help, go to your network adapter settings. On Windows: Control Panel > Network and Sharing Center > Change adapter settings > right-click your connection > Properties > Internet Protocol Version 4 (TCP/IPv4) > Properties > Use the following DNS server addresses. Enter 8.8.8.8 (Google) and 8.8.4.4. Apply and restart your browser. This bypasses your ISP's flaky DNS entirely.
Clear browser DNS cache. Chrome has its own DNS cache. Type chrome://net-internals/#dns in the address bar, click "Clear host cache." Then visit chrome://net-internals/#sockets and click "Close idle sockets" and "Flush socket pools." Restart Chrome.
Reset the network stack. Open Command Prompt as admin and run: netsh int ip reset, then netsh winsock reset. Reboot your PC.

Alternative fixes if the main one fails

If the above didn't fix it, try these in order:

Disable IPv6 on your adapter. In the same network adapter properties window, uncheck "Internet Protocol Version 6 (TCP/IPv6)." Some routers handle IPv6 DNS poorly, and this forces everything over IPv4.
Check your hosts file. Open C:\Windows\System32\drivers\etc\hosts as admin (Windows) or /etc/hosts (Mac/Linux). Look for any stray entry for the site you're trying to reach. If you see one, put a # at the start of that line to comment it out and save.
Temporarily disable your firewall or antivirus. I had a client whose AV software inserted a DNS filter that blocked all sites with "download" in the URL. Turn it off for 30 seconds to test—if the site loads, you've found the culprit.
Use a different browser. If the site loads in Firefox but not Chrome, it's a Chrome-specific cache issue—repeat step 3 carefully.
Reboot your router and modem. Unplug both, wait 60 seconds, plug back in. This flushes the router's own DNS cache.

Prevention tip

Once you've got it working, don't rely on your ISP's DNS. Change your router's DNS settings to use Cloudflare (1.1.1.1) or Google (8.8.8.8) permanently. Go to your router's admin panel (usually 192.168.1.1 or 192.168.0.1), find the DNS settings under WAN or Internet, and set primary to 1.1.1.1 and secondary to 1.0.0.1. This prevents 90% of DNS_PROBE errors before they happen. Also, schedule a weekly DNS flush using Task Scheduler on Windows or a cron job on Mac/Linux—just a simple ipconfig /flushdns script. I've got a client who does this and hasn't seen the error in two years.

Real talk: this error is almost never your fault. It's your ISP's crap DNS or a stale cache. The fix above covers both.