There are several ways to see what is the cause of your network issues.
Network issues can be caused by various reasons. Best way to figure out what is the core problem is to try the following steps.
Step 1: Reboot your devices
Before you start spending 30 minutes on troubleshooting something that could be fixed with a simple reboot its best to get it over with. Turn everything off and on again. That means your computer or the device that your are using (phone or tablet) and also your router.
Wait 60 seconds before you turn them on again as then you can be sure the router has cleared its caches.
When you have restarted then try getting online again. Try other devices as well, if no device can get online then likely it is an issues with your router or your ISP (internet service provider).
Step 2: Check physical connections
If your network problem persist after rebooting you should check if your are physically connected.
If you use an Ethernet cable to connect to your router, make sure that it is not unplugged or damaged. If there is a wireless setup, make sure you it is not set to the off position.
Once your have made sure that you have a proper connection, check your equipment. Do the lights flash green as normal? If there are no lights then the device could be dead or malfunctioning. However if you get red lights, or a power light but no connection light, your ISP is likely experiencing disruption.
Step 3: Contact your ISP
If you have completed the above steps successfully and still you are experiencing network issues you might want to contact your ISP provider.
Step 4: Wait the networks issues out
Once you have contacted your ISP provider and let them know that you are experiencing issues, all you can do is to wait. Sometimes you just can not fix these issues on your own.