I've lost IMEI no in my NOKIA phone.

I've a NOKIA 1108 phone since 2006. It didn't create any trouble for me ever. Actually it was a no problem phone. But last week (Most probably 28/02 or 01/03 when I switched on my phone, I got a massage " SIM not valid". Then and there I placed that SIM on my anothe cell phone, and it works.....

Then some one told me to check the imei no of my phone set (NOKIA 1108), and I was shocked to see that there is no imei no. only some question marks and a 4, i.e. "??????????????4" are there. Now, my first qutiestion is, " Is it possible to change a imei no like tat?". and the sccoend is "How can i get it back?".

Thanks and regards.

Shirshendu