The IP connection between your device and the router or between the router and Internet can get hung up. A restart of the router reboots all its systems, including the network processor and wireless radios.
If your router has a reset button, hold it down for a few seconds. If not, restart it by removing the cable from the power socket, waiting half a minute and then plugging it in again.
If that doesn't work, check to make sure the router firmware is up to date. Look for the update option under "System" in router software you have installed on your computer. Only download router firmware updates from the manufacturer's website.
You may also find that resetting and reinstalling your router software may do the trick. For most routers, this is accomplished by holding down the reset button and then reinstalling the software.