Updated
Quick fix
Turn Wi-Fi or Bluetooth off and on, then “forget” the network or device and connect again. Restart the phone and the router or accessory. Resetting network settings is the last step and erases saved Wi-Fi passwords.
Step by step
Toggle it off and on
Turn Wi-Fi or Bluetooth off, wait 10 seconds, and turn it on again. Turn Airplane Mode on and off as well.
Forget and rejoin
In Wi-Fi settings, tap the network and choose Forget, then join again and re-enter the password carefully. For Bluetooth, forget or unpair the device and pair it again.
Put the accessory in pairing mode
Headphones, speakers and cars must be in pairing mode to show up. Check the accessory’s manual; it may already be connected to another phone or laptop nearby.
Restart everything
Restart the phone, and restart the Wi-Fi router or the Bluetooth accessory.
Check other devices
If no device can join the Wi-Fi, the problem is the router or internet, not your phone. See restart your modem and router.
Update
Install the latest system update. Car and headphone makers also release updates.
Reset network settings
iPhone: Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings. Android: look for a network settings reset in Settings, often under System or General management (names vary by manufacturer). This erases saved Wi-Fi networks and pairings, so you’ll need to reconnect.