hi manisha:
thanks for the detailed error report, and welcome to the forum community. (if you have a moment, please consider posting a hello on the introduce yourself thread!)
it looks like the error code here is 503, and you are using app engine, and the process used to work. usually, this combination of clues indicates that you have run into your usage quota on app engine. i'm guessing it works again now, or if it doesn't you just have too much data in and out and the free app engine tier may not be enough for you anymore.
you can find more information about the quota limits here, and in general if you search this forum for "quota" you will likely find other people who have run into this problem.