That triangle is your cellular connection icon. (To its left is your WiFi indicator and to its right your battery charge indicator) Typically it would be filled or partially filled to show an approximate level of cellular signal strength. That X isn't a good sign however. Try re-seating your SIM card. If that doesn't make any difference, log into your online cellular service account and check that your status is still paid up and active.
If all that checks out OK, it could be the SIM card has failed. Contact your carrier to get a replacement.
Your profile here indicates your carrier is Sprint. If that's still current, try looking in your Settings menu for a 'Carrier settings' option. Open it and there could be an 'Update PRL' option and a 'Update device config' option. Do both and see if that makes that X go away. (That 'Carrier settings' menu will most likely be buried in a submenu somewhere, and it will vary depending on your phone model. As an example, with one of mine it's in Settings >> Wireless & networks >> More >> Cellular networks >> Carrier settings)
Or as a last resort, try going into your Settings menu, find an option named 'Network settings reset'. This resets your cellular, WiFi, and Bluetooth settings all back to their defaults. But keep in mind this relates to all network related services in your phone so things like saved WiFi networks and saved Bluetooth pairings will get wiped too.