Why can’t my iPhone be connect to cellar data ?

My phone has been working just fine since I got it . Today, randomly my iMessage was shut off until I found a way to activate it back on . After my cellar data was having problems . My bill is paid,but my internet isn’t working . I keep getting a sign about an error occurred and to activate to cellar data . The cellar data is on . I keep turning my phone off and on but it still won’t work . What should I do ?