It sounds like maybe the configuration of the router has been changed somehow.
You should be able to call your ISP and have them watch their equipment to see if they're even receiving the call from your router. If the call isn't getting through to them at all, then it may be either the configuration having been screwed up somehow, or the router being damaged. If they're seeing the call but the connection isn't being completed, then it could be the same things. If the connection gets made, but you're never assigned an IP, they should be able to check whether your router ever sends authentication information. (Assuming your router doesn't have a manually set static IP address for the WAN side.) The connection will probably normally time out and disconnect if authentication isn't passed within a short time.
I don't remember enough of using Ascend routers to know for sure where to look for errors. If you telnet into the router during the dialout, you should be able to see a large box on the left side of the screen, and 8 small boxes on the right side. One of those boxes will have 3 rows labelled D, B1 and B2 (I think). To the right of each label, is normally a dash when it's disconnected. The D channel's status should be an asterisk I think, indicating that the D channel is connected, but I'm not real sure with a non-dedicated ISDN line.
When the dialout happens, the B1 channel's status should change to a small letter d. When the connection is made, it should change to an asterisk indicating the connection is made. If it then changes back to a dash immediately, then for some reason you're being disconnected from the other side.
One of the other small windows may show status messages. I think you should see something like "M31 Call Disconnected" or something like that. The M31 indicates the message number 31. You can hit the tab key to move to different boxes, and the down arrow when that box is highlighted will scroll the messages. One of them may give you a cause code or error. If you don't see the message window at all, you can go to any of the smaller windows, and the left arrow key will back through the options for what can be displayed. You should eventually be able to find the message log window.
Windows Hyperterminal sometimes has problems telnetting into Ascend routers. You may want to use some other product like SecureCRT, or a command line telnet program. Also, connecting via a direct connect with the serial port will work too, you just set the terminal program to do so. Telnet is just easier to do.