@Dadam27: To begin with, please press the Volume Down and Power buttons together for around 10 seconds until the phone restarts, as this will refresh the device and address any temporary software issues that might be a contributing factor. Don't worry this won't affect your personal data or settings. Now, if possible, please unplug your router at the wall for 30 seconds, then plug it back in and wait for the calibration process to complete.
Following these steps, please head to Setting > Connections > Wi-Fi > On > Select your router from the list of available connections, and fill in the required fields. If you have previously established a connection to the router, please tap the cog symbol to the right of the router, select 'Forget', toggle Wi-Fi off, then on again, then run through the steps as laid out above.
If the problem persists, please head to Settings > Connections > Flight mode, and make sure that this is toggled off. You may also wish to head to Settings > General management > Reset > Reset network settings > Reset settings, however please be aware that this will reset any personal settings for Wi-Fi, Bluetooth and Mobile Data. I also recommend attempting to connect to another Wi-Fi router, a friend or family members, as this will help to determine if the problem lies with the phone, or the router/network itself.
