So I was coming home from my village in the car, we pass by a very weak signal area, and like usual, all the phones' signals were gone and the radio stations stopped functioning properly. When we reached the nearest city, all the signals were back up except the one on my N9. Since then I wasn't able to use my SIM Card at all, and when I try checking the IMEI code it doesn't show up either. So when I came home I tried my SIM in my dad's iPhone 4G and it gave "No Service", I went and bought a new pre-paid SIM Card and it still doesn't work, they said I need to go to the airport to unlock it because it was locked on my previous number. So I went to the airport today, "unlocked" it as I doubt they unlocked it, I called my operator provider, they told me that within an hour my new SIM should work.. My N9 pops up with a "Enter SIM code" prompt every 5 mins and "SIM Services" shows up for 2 secs and then goes away directly. Still nothing, and when I check "Call" and "SIM Card" settings it gives me SIM Card Error.
Now, what to do?
P.S I swear to God I am serious about this, please help.

When my N900 started dieing because a faulty "something" in the GSM chip it would start prompting me a simple toast message saying "General Error" when trying to make a call. This was simply fixed by a reboot. This error I was getting about once a week, but without any crossed-out SIM icon. I even went out of town in a no-signal area and it would recover okay. Than, the problems started getting worse. After a while that yellow toast message saying that all the telephony functions are bla bla... started getting even more, and more often. I was able to fix that by shutting-down the phone and by switching the SIM with another SIM from another operator (sure, this was pure placebo) and then replacing it back with my SIM. When it completely died, before using the magic soldering iron and killing it beyond repair state, I checked and I had the same problem with the detection of the IMEI number.

If by any chance you are not lucky, and this is a GSM chip fault, I am sorry, but I think a repair is not more profitable than getting a new N9 device.

That's what I am afraid of. I hope nothing is wrong with my Hardware because I seriously can't get a new phone right now. I will wait for tomorrow and see what happens, I am seriously worried about this!

Take it to a local service for diagnosis. This is the best you can do. You can try a reflash. But that's about it.

Test it out by forcing the SIM card in, with a piece of paper to stay very fit. If it works, it may be your SIM reader, if it's still not working, I'm afraid I have some bad news for you.

I tried reflashing both eMMC and rootFS but to no avail. I took it to a phone store and they said just like my operator provider said that I need a new SIM and register it in some place at the airport and request my operator provider to activate my SIM. So I did, and after waiting for 1 hr of ******** nothing happened, called'em and they said that I may need to wait 24 hrs due to pressure on their tasks. Now, after a reboot with my new SIM, the IMEI code DOES show up and when I wanna check my credit, it tells me “SIM Card not ready”. And regarding the old SIM, I did try putting a thin paper between my SIM holder and the SIM Card, but nothing happened. I think I will wait for tomorrow, as only time can tell.

You can test this by using your dad's sim card, or anyone from your family who already got a activated SIM card. I had the same problem this time with my HTC but it was due to a high amount of internet traffic on a factory-faulty sim chip and just a new sim fixed the problem.

So what exactly is your problem, SIM ERROR? Can it detect that the SIM Chip is dead? Don't you have an old SIM that you can test? The best way to test it is to insert the SIM in another phone, encrypt it with a PIN, get it out. Now insert it in your phone, if you are prompted for the PIN the GSM chip + Sim reader are working fine, if not...

My new sim is already encrypted with a PIN code and whenever I start my phone it asks for the PIN code of my SIM.

Same happaned to me when I was in a train. N9 lost singnal and then didn't found it even when we were in the center of a city. I put my sim card to ZTE Blade and it didn't find network either. I did manual network search in ZTE Blade and that worked. After that sim card worked in N9 also.

I really hope it's just a burnt SIM Card. Because when I tried it in my dad's iPhone 4G, it gives "No Service", even after a reboot. And it doesn't work either with my aunt's Galaxy Nexus. And my dad's SIM Card didn't work on my phone [BUT] maybe because the new operator system in my country made my phone locked on the previous number so I can't access any other. So that's why I had to take it to the airport to unlock it again. If nothing happens after 2 PM tomorrow, I can know whether it's the phone or the SIM's fault.

See if any other registered card works with your N9, that's the simplest thing you can do to check if it's N9's fault.