SIM900 GRPS Shield Power down over issue AT+CIICR cmd(Bring Wireless Coms)
The SIM9000 shield requires up to 2 Amperes to operate its wireless functions. Thus it must be powered from an external 9 Volt power supply, and the supply should be able to supply at least 2 Amperes, preferably more. 

The shield will thus not work if powered through the Arduino, or using a low current external supply. The current draw for initial setup is low, hence registration of the SIM might be fine. Enabling wireless pushes the current draw beyond what your supply can cope with. This is shutting your setup down.

Replace (or use) a 2 Amperes or better 9 Volt DC power supply, feed this power to the shield, not the Arduino, and attempt your code again.

Note that there are some changes required in using an Arduino Mega instead of the Uno with the TinySine GPRS shield, which are described in the help files and on some example videos on YouTube.