There are a number of reasons your phone might not be connecting to its network. First and foremost, you might not actually be in an area with reception. Being in a rural area, below ground or deep within a building can restrict signal. Your carrier might be having issues providing coverage in the area or the network might be down; if you contact them over social media they might be able to let you know.
Signal issues can arise if your phone’s software is out of date too. If you’re having problems, try updating your device’s software to see if it makes a difference.
Phones are complex and can sometimes go haywire. The device might just need a reset to kick it back into life so try turning it off and on again.
A damaged SIM Card can cause signal issues as well. Remove the SIM Card and inspect it for damage. Try using the SIM Card in another device or a friends’ SIM Card in your device to establish whether this is the problem. If the SIM is the problem, you need to speak to your network provider.
If all else fails, there could be a problem with the device itself. Take it back to wherever you bought it or contact the manufacturer so they can take a look at it.