not running program while power on after sametime

i have code about sim900 gsm module with arduino uno.
when i program the code, it is working properly.
after sometime when i power On the system, it is not working. i have to put the code again and again due to this problem.

pls give the answer for solve this problem,
thank you

Post your program so we can see what you can see.

And please use the code button </>so your code looks like thisand is easy to copy to a text editor