Could be a lot of things.

You very, very rarely (read "never" but I won't be that dogmatic) connect directly to anything on the Web or on the Internet. When you push the "do it" button, your data packets are routed (by a router!) to a computer ("node") that sends them to another node, etc. Each stage of the journey from node to node is called, technically, a "hop" (really!). Each hop is determined by the sending node, which determines the "best" route for the data packets to take. If your message is long enough, it's broken into pieces and then reassembled at the other end (and how this is done is too complicated to explain here, but it's on the Web and you can search for the explanation yourself if you're interested).

For instance, if I send a data packet from Pocatello, Idaho to Idaho Falls, Idaho, it usually takes about thirteen hops -- to cover a road distance of about 50 miles. (Ain't the speed of light wunnerful?)

Thus, a problem at any point in the journey can cause problems in sending your data packets. In brief, the 'Cat needn't have problems for you not to connect.

If you'd like to see what nodes your packets traverse, download a program called "traceroute" (there are several different varities), install it, and from "Run" enter "traceroute www.mudcat.org" (or whatever your target destination is).