Why has my internet speed suddenly decreased?

[969]

I got my internet connection a few days ago, and since then i have got an average speed of 100 to 110 kb/sec, but today morning, the speed suddenly out of nowhere was halved, it gave an average speed of 55 to 60 kb/sec. Why did it suddenly decrease? It is so damn annoying! Everything i download now takes double the time it used to take before and it is affecting me. What is the reason and solution (if any) for it?
There is some additional details i would like to throw it, hopefully it would help you guys! I am an Indian, and i get my internet services from a telephone modem (BSNL), not a USB modem (like MTS). And yeah i have downloaded much in the last two days, around 5 Gb maybe, so will that affect the internet speed? And also another thing i forgot to mention, the 'internet' LED in my modem went out, i restarted the modem and the file which i was downloading, that is when the internet speed was halved.