Arduino not responding.

I'm working with an Arduino Uno, GPS and SD card modules. The arduino hangs while some values are fetched from an SD card.

Here the interesting thing is I've finished my project before and till yesterday, it works perfect. I got another error for sometime that it resets automatically when hangs.

I'm double sure about my circuit and program are the same when it is worked properly and now. It might not be the common errors like memory overflow, stuck in loops as it worked perfect earlier. I also checked the modules independently (both GPS and SD card). I checked everything else (like finding the exact point by using some extra Serial.prints for identification) and couldn't find the error. So please help me...