Figuring out why you lost your connectivity is often a matter of following the trail of evidence. The tools you use to follow that trail are not that hard to use. I found a good tutorial on troubleshooting internet connection issues over on Jeff Atwood's Coding Horror blog. One of the topics he discusses very thoroughly is bandwith. From the post:

How many kilobytes per second does your ISP allow you to transmit and receive? Most consumer internet connections are highly asymmetric-- they offer extremely fast downloads but only a tiny fraction of that for uploads. Your typical cable modem is around 300 kilobytes per second when downloading (sometimes even faster), but only 30 kilobytes per second when uploading.

He gives pointers to several free tools to help with your analysis.

Recommended.