Fortunately, most of the time, fixing a problem with your phone's signal is simple and only takes a few seconds. Other times, you will have to take more drastic measures.
Turn airplane mode on and off.
iPhone: iPhone users can swipe down from the top right corner, older iPhone models swipe up from the bottom of the screen, and hit the Airplane Mode icon. It will turn orange when enabled. Again, please wait up to a minute before turning it off.
Restart your phone
Our phones are like miniature computers, and like computers, sometimes you can fix problems just by rebooting them.
iPhone Users: If your iPhone has a home button, you can press and hold your iPhone's sleep or wake button until the power slider is displayed. Drag the slider to the right. Once the device shuts down, press and hold the Sleep / Wake button until you see the Apple logo.
Take out the SIM card.
Another troubleshooting step is to remove and then put the SIM card back into your phone with the phone turned on. For doing this, you will need a SIM card tool, usually included in your phone box, or you can find an open paper clip to remove the phone's SIM tray.
Sometimes unexpected signal problems can be traced to problems with your wireless provider. A cell phone tower could be down, or the tower's fiber optic cable could have been cut, causing the outage.
Other times, a new signal problem may be due to a defect in your phone or in your SIM card that is not working properly. Contacting your carrier after trying these measures above is the next step in solving your signal problems.