My internet data keeps disappearing even when I don't use the internet?

My iphone screen cracked and i had to send the phone away to fix it (not Apple). I forgot to remove the sim card before sending it away (if the people want to use the money or data or something from my sim card, you know people these days). Before i sent the phone away I had 26MB data left. After i got the phone back it still had 26MB of data. But 3 days afterwards i got a message from my carrier stating that all my data and airtime have been depleted, even though i did not use any because my cellular is permanently turned OFF because i only use Wi-Fi. I never call or send messages. Now everything is gone! When my phone was sent away i put a passcode lock on but of course they could only put the simcard in a different phone. I doubt that the people got anything to do with this because i don't know how they couldve done it. What is a easy explanation for all of this? Today i bought another 30MB data to use at school the next day but it is disappearing also. Cellular is turned OFF always

Just go pay the $5 or so to get a new sim card. If it's hacked it's hacked. It doesn't matter who did it, you still have only one way to stop them and that is to change your sim card. Now if you want to catch them then file a report with the police and the phone company you use and they will investigate, but either way, get the sim changed to stop future theft.
Most theft is insiders. That is why stores have most of their cameras watching the cashiers. The persons who steal the most are probably the managers because they have the ability to blame it on someone else.

But like I say, it doesn't matter who stole. The best thing you can do is change the sim. Everything else is optional.

My phone has 2 places to turn off the data. One is for data and the other is for automatic updates to my phone and apps. I have the data turned off in both places. I have a Android phone. I do not know if Iphones have 2 places to turn off your data, but you may want to look into it, because this could be the reason you are losing all of your data.