SIM800A NOT WORKING AFTER 1MONTH(1st one month established network successfully)

Hai Sir,

we designed a custom board...In that we integrated atmega2560 circuit and sim800A circuit ....In that customised pcb,we are giving 12v/2A as input to pcb and next we regulating 12v to 5v using Lm2576s-5.0 regulator then we are giving that 5v directly to gsm sim800A....

"first 1 month it worked perfectly means established network and sent data to the server.....but now it is not taking even at commands and not establishing network....connections and code everything are perfect....we didnt change...but that pcb is not working properly....

can anybody please suggest,wat was issue here...how to solve this pronlem?

Thank you inadvance :)