Hoo boy, so many possibles. In control panel, modems, is your modem listed and can you do a "modem diagnosis" from that window? If no, is it listed in device manager (rt click my computer, properties, device manager)? Right click your ISP icon in dial up networking, properties and is modem selected? Try dialing out with hyperterminal (might try this first). Hopefully you'll just need to uninstall dial up networking and delete your ISP connection, then start, run, type inetwiz - this reinstall DUN and setup a new ISP connection. All the above can be done in a few minutes. If no luck, remove modem in device manager and reinstall. Cross your fingers it's just a windows snag. Did you add anything programs, etc before the problem began? Just an afterthought.
Is it saying modem not detected, or no dial tone, and is there a DUNS error code with the message?