The problem is often just that the WiFi router needs to be re-started. The next time you find you have no internet, consider this as one of your first 'ports of call'. Your router will usually have an on/off button on its back. Either turn it of using this button, unplug the power from it or switch if off at the wall. Wait a little while (eg 10 secs) then plug it back in/turn it back on and wait for all the lights to come back on and show green (sometimes blue as well).
Then recheck your internet/wi-fi connection from the device. Perhaps try turning off your WiFi on the device and turning it back on, then see if your WiFi network appears in the list now. If you don't get automatically connected to it, select it and re-join it.
Hopefully this will work in the majority of cases. There are times where you will find that your internet provider is actually doing maintenance or has a problem in your area. If you are still having problems, this may be the case - I usually use my smartphone to check for outages in the area via my ISP's website, once I have ruled out any problems from my end.