It means that your phone and router/modem or any other network device are using the same phone line - being on the internet and talking on the phone at the same time isn't possible in this case of scenario.
To fix it you need to split your phone line into 2: one that goes to the networking device and the other one should go into the phone.
Picture of phone line splitter:
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As you didn't have this problem before the only reasonable cause could be that someone unattached your phone line and put it back into the wrong port
Can you describe your network/phone configuration a bit - what cables and devices?
Also calling your ISP is a good idea as other people here have suggested as they know your network configuration better than we do.
This post was edited by xHanzx on Feb 11 2010 10:07am