Check the signal strength in the status bar to make sure your device is connected to a network. If the signal is weak or you have no signal, move to an open location free from obstructions, or get close to a window.
Check the device memory and memory card to see if memory is available. If the memory is full, remove messages or other files to free up space.
To view the status of your memory
From your Home screen, tap the "Settings" icon.
Find and tap "Settings" > "Storage" to view the status of your memory.