As with any networking issues, there are 2 places to look at first: the networking equipment (router) and the device having the issue (computer, smartphone, tablet etc.). As a first step, restart your networking equipment and device and see if that fixes the problem. As with the router, network, wifi, and software problems often get fixed automatically with a simple restart. If it doesn't help you, check with the following:
Check Modem Lights
Flush your DNS
Change your DNS server address
IP address conflict
Outdated network driver
Check your ISP's status