Quick fixes:

Restart: Turn off Wi-Fi and Airplane mode, then power off your phone for 30 seconds. Restart and turn the Wi-Fi back on.

Re-connect: Tap the network name again and re-enter the password (don't copy-paste!).

Move closer: Your router might be out of range. Get closer and check the signal strength.

Diagnosis:

Isolating the problem: Try connecting another device to the Wi-Fi. If it works, the issue is likely with your phone. If not, the router might be the culprit.

Phone troubles: 

Forget and reconnect: Delete the network from your phone's memory and re-add it. 

Safe mode: Restart in safe mode (no third-party apps) and try connecting. If it works, an app might be interfering. Uninstall recently downloaded apps one by one to find the culprit. 

Reset network settings: This wipes all Wi-Fi, Bluetooth, and mobile data settings. Back up important info before trying.

Router woes:

Restart the router: Unplug it for 30 seconds, then plug it back in. Wait for it to fully boot up.

Check router settings: Ensure the network is broadcasting and hasn't reached its device limit.

Advanced tricks:

Update software: Make sure your phone and router have the latest updates installed.

Check channel interference: Consider changing the Wi-Fi channel on your router to avoid interference from other networks.