Being unable to connect to Wi-Fi is a common issue with the Lenovo B575-1450. First, before you do anything, make sure you have Wi-Fi enabled. On your ask bar on the bottom right corner f your screen, click on the internet icon, and make sure your Wi-Fi is enabled.
If it is not, then maybe your drivers are out-dated. Consult the [http://support.lenovo.com/us/en/products/Laptops-and-netbooks/Lenovo-B-Series-laptops/Lenovo-B575-Notebook?tabName=Downloads&linkTrack=Mast:SubNav:Support:Drivers%20and%20Software|Drivers%20and%20Software&beta=false] for the B575 for drivers for your respective operating system.
Lastly, if neither of those solutions work, your Wi-Fi card may need to be replaced.
For any other issues you have with the Lenovo B575, try taking a look at the [[Lenovo B575-1450 Troubleshooting|troubleshooting page]]