Most likely, the phone company didn't do a decent job with the wiring, and/or changed wiring that is preventing you from attaining a higher speed connection.
One thing that happens is called "changing the pairs". This basically means that the copper wires/connections that your house previously used have been changed outside on the pole. This is a permanent thing and is used to fix broken phone service.
However, what sometimes happens is that you get switched to a very old set of wires or connections that don't service high-speed modem connections very well.
A 56k modem connection is dependent upon a number of things.
1. You need good quality wiring and terminations (where the wires connect to the boxes) in the house and outside the house on the pole. In short, all the copper between you and the ISP (Internet company) needs to be a certain grade/age/quality. I am also aware that distance can affect you getting a 56k connection, but if you got better speeds previously, this isn't the case.
2. You need availability of 56k service at your ISP. Its possible that the internet service provider has no more 56k modems available as they are busy already servicing connections. You should call them, and hopefully their tech support will give you a decent answer.
If I were you I would call the Telco and plead your case. Most likely they have switched you to very old copper lines and can probably make the switch to "another pair".
If that doesn't work, you can always sabotage your own phone line and tell them you have no dial tone!!!!!