Dunno if I should post here...

I have a domain reged with a registrar and a hosting my web somewhere else...recently when I trying to access my website by that domain, it returns as Cannot find server/DNS error. (it used to work) But when I type in IP instead, the site works all right. Ping and Tracert by domain works OK too (it recognizes the IP associated with the domain). Typing in DNS address and they work as well. I double checked the DNS info on the registrar and all things are put correctly.

My question would be what will be the possible cause of the problem? on the server side or the registrar side? I've with my old registrar for three years haven't had such problem...help would be greatly appreciated. Thanks! :)

Your host should be able to fix that. You should contact them.

Maybe it was up for renewal and you didn't renew it?
Do a whois on your name.
This will tell you the nameservers and other info that will give you a clue.
Once you have an idea, or even if after a whois you don't, contact your registrar.