If you're using dial-up, I can only think of 3 reasons why you should keep losing a connection:
1. A fault with your phone line. (Get your service provider to check that the gain is set to maximimum).
2. A problem at the ISP's end of the connection. (Unlikely if you're the only person reporting the problem).
3. Your dial-up networking settings have become corrupted. This is probably the most likely cause. To check, go to your dial-up networking folder. (Depending upon your version of Windows, the route is probably something like Start > Settings > Dial-Up Networking or Start > Control Panel > Dial-Up Networking). Right-click the icon for your ISP. Select 'Properties'. Click the 'Dialling' tab. Now look at the place labelled 'Enable idle disconnect'. There should NOT be a tick next to it. If there is, click to remove it. (If it wasn't there anyway then, sorry, it's not the cause of your problem). Click 'OK'.
If the above solution doesn't work, you could also try running System Restore to take your PCs settings back to a date before the problem started to occur.
Hoping something here helps,
Chris