DIAGNOSIS The 5GHz radio might have crashed or been disabled. It can happen after a firmware hiccup or a bad settings change. The router is still running, but half its wireless is offline.
FIX Reboot the router. Unplug it from power for 30 seconds, then plug it back in. Wait a full 2-3 minutes for it to completely restart. The 5GHz network should reappear. If it does, avoid changing any advanced wireless settings right away to see if it's stable.
IF THAT DIDN'T WORK You'll need to log in via Ethernet cable (since you might not have 5GHz Wi-Fi). Go to Advanced > Wireless > Wireless Settings. Verify the "Enable Wireless Radio" box is checked for the 5GHz band and that the SSID isn't hidden. If it was disabled, re-enable it and save.
HEADS UP Some regions have DFS channels that can cause the 5GHz band to automatically shut down if it detects radar (like from weather stations). If you live near an airport or government facility, try manually setting the 5GHz channel to something like 36, 40, 44, or 48 in the wireless settings to avoid this.