First try restarting your router/modem. Once it is fully up and running, and you are sure its WiFi is turned on...
Try checking the channel setting on your router. Go into the routers "configuration" and then go to the "Wireless" option. In here look for an option which deals with "Channels".
If the router is set to "auto", you should usually see which channel your router is using.
Next you need to check the channel setting for your Laptop:
1. Go to "Control Planel" and then "Network and Sharing Center".
2. Click on "Change Adapter Settings".
3. Now right-click "WLAN device" and and then click "Properties".
4. Go to the "Networking" tab and click on "Configure".
5. Next click on the "Advanced" tab, go to "WZC IBSS Channel Number property" and look at the value/number it is using.
6. If this value/number is different than the one that your router is using, this can cause your laptop to not connect correctly.
You can look into switching to manual selection of channel on the router and then select an appropriate channel which is supported by both your router and your laptop.
If this doesn't help... then try the following...
First, make sure your laptop is 'not' connected to the modem/router via an Ethernet cable and follow the steps:
1. Go to "Control Panel".
2. Next go to "Network and Internet" and then "Network and Sharing".
3. Now click on "Manage Wireless Networks".
4. On this screen, remove all wireless connections by "right clicking" on it and select "Remove network" from the drop down menu.
5. After doing this, close the open windows and reboot the computer.
6. Once the computer is fully reloaded, scan for the "Wi-Fi networks".
7. Click on the available network that you want to access and select "connect".
8. It takes a moment, but then you will be asked to give a password for your Wi-Fi connection. Type in the password and click the "connect" button.
It should now be connected.
NOTE: If this doesn't work, you may want to reset your modem/router, and then once it is fully loaded again, repeat the above steps.
If this doesn't work, test seeing if your laptop will connect via Ethernet. If so, then your WiFi card may be going. You can either look to replace it/have it replaced, or get an external (plugs in via USB) WiFi card.