Before you start
Do not factory-reset your router yet. A reset removes settings and is very different from a restart.
Check whether another phone or computer on the same Wi-Fi can access the internet. This immediately tells you whether to focus on one device or the wider network.
1. Check another device
If other devices work, concentrate on the affected device. If nothing works, check the router, internet connection and ISP status first.
2. Reconnect to Wi-Fi
Turn Wi-Fi off and on, reconnect to the correct network and confirm that airplane mode is disabled. If necessary, forget the network and reconnect using the correct password.
3. Check your IP address
On Windows, open Command Prompt and run ipconfig. A 169.254.x.x address often means the computer did not receive a usable address from DHCP. A normal private address such as 192.168.x.x or 10.x.x.x means you can continue testing.
4. Test the path
First test your default gateway. Then test an external IP address. If external IP connectivity works but website names do not, investigate DNS rather than repeatedly restarting the router.
5. When to get help
If several devices fail, router status lights indicate a WAN problem, physical cabling is damaged, or the fault returns frequently after basic troubleshooting, contact your ISP or an IT professional with the observations you collected.