Iphone has really bad service unless connected to Wi-Fi?

Since saturday, my phone hasn't been working unless i've been connected to Wi-Fi. At home i have Wi-Fi so its not really a huge issue, just an inconvenience. I can still text and call, but imessage doesn't work, and neither does safari or twitter or facebook or anything that requires internet. When i'm not connected to Wi-Fi, instead of having the Wi-Fi signal it says LTE. Any idea what could be wrong?

You went over your maximum data usage.