Sounds like a problem with your unspecified web server. If you were to tell us what you were using (eg Apache 2.0.19 on Windows XP Pro SP3), and ideally your DynDNS hostname, we should be able to help you (or at least tell you where to get that help).

Edit

I'm using windows server 2008 R2, IIS 7.5. I need to restart my modem if the problem occured. And also I'm using two domain name on this server, will it cause this happen?

Only the modem, or the modem and the PC?

If it is just the modem then it sounds like a problem with the modem (it could be a problem with the update client built into it if that's what you're using). If the computer too then I'd suggest you ask on the Microsoft IIS forums.

Modem and PC, but the weirdest thing is if I restart my modem it will work normal for a few hours, so now i dont know which is the problem, I use the software client to update my domain.

Whatever the problem is, I don't think it relates to DynDNS. Do you have any other problems when this happens?

Right now I'd lean towards a problem with your modem or ISP, but it could also be a problem with your computer. The next time it happens use this tool to get the complete results of viewing your webserver remotely and post (edit your question) the complete output.

Edit

That suggests a whole range of options, from your WAN IP having changed (but your DynDNS hostname not having been updated) to issues with your Windows PC. I'd suggest that you work through this guide the next time it stops working (before you reboot anything) and if that doesn't identify the fault post the information listed at the bottom of that guide.