Here are a few possible reasons for your problem and some solutions:
1. Check the cable connecting the modem and the network card of your computer, make sure it is properly connected and the cable firmly attached to its connectors.
2. Your network card might not be functioning properly, this is due to several possible reasons:
- Your network card is disabled, you can enable by right clicking "My Network Places" then choose the "Properties" tab. Then you'll see a list of your network connections, if you see a disabled connection you can right click on it and select enable to get it working again.
- Your network card's device driver is not installed. You will have to find the driver installer for your network card if windows xp doesn't support it.
- If your network device is built-in to the mother board, check the cmos set up if the network device is enabled. Also check if the network driver of your motherboard is installed.
- Your network card might not be configured properly, this usually happens with computers using older operating systems. You'll have to configure it manually and require you to have knowledge of networking stuff.
-Your network card might be damaged and needs to be replaced.
3. Your modem might be damaged or is having a problem.
4. If you have a router, then its possible that your router is not configured or was reset to factory default settings.
There are more possible reasons for your problem, I only cited the common ones. I hope you get it fixed soon.