My phone is behaving weirdly (Urgent!), can I get some help?

A

AC Question

My phone is behaving weirdly (Urgent!)

Sorry for my bad English, I am not a native speaker.

Last week, my ZTE Blade L2 started behaving very weirdly.
I didn't install any app at that time, didn't disable any stock apps nor downloaded anything. My phone's not rooted either.

It all started with not being able to receive or make calls.
My sister called me and everything that appeared on my screen was a "Unfortunately the process com.android.phone has stopped".
A few days later the phone stopped detecting my SIM card. It took my three or four hours to get it to work.

At this point I thought resetting the phone was a good idea. Silly me.
When I turned it on after resetting it, there were a lot of messages like "Google Play/Google Music/etc has stopped working".
The stock keyboard didn't work at all and I had to install Google Keyboard from an APK, even after that it took me a while to fix it because it also kept crashing.

Now I can't even use my launcher of preference (Smart Launcher 3) without it sometimes crashing and all my apps crash 95% of the time when I open them (including Settings)
I can't make calls either; now it never detects my SIM card and I can't use any app.

I need help ASAP!
I put a "Urgent" in the title because I still go to high school and my mother tells me to send her a message when I arrive to class (I go there on a bus and she's very paranoid).
She's getting angry with me because she thinks I installed a virus or something.

Thanks in advance!