GSM/GPRS modem problem command AT+CREG replies (0,2)

GSM/GPRS modem problem

Hi,

When I configure my modem for GPRS communication I do the next steps.

AT+CSTT="APN","User","password"
OK

AT+CGATT?
+CGATT: 1
OK

AT+CIICR
OK

AT+CIFSR
10.8.144.163

AT+CLPORT?
TCP: 2020
UDP: 3030

OK
AT+CIPSTART="TCP","10.8.144.163","2020"

I was connected with another modem via GPRS, everything was good but in my
office the power down and my GRPS modems turn off.
When we recover the power in my office I turn on the GPRS modem and
all of my configuration lost.

But the lost configuration is not my problem, the main problem is when
I turn on the GPRS modem, it does not connect to the GSM Network.

When I use the command AT+CREG?, it replies with (0,2) .... so it is
not registered with the network.

I dont understand what is happening...
Can anyone help me????
My GPRS modem is SIMCOM