I am assuming that you are using DreamWeaver so try recreating the site cache in DreamWeaver - Site > Advanced > recreate site cache. Close DreamWeaver, reopen the .dwt file & see if that cures the issue
As far as I recall I don't think the CoffeeCup software supports the .dwt file extension, although it's a good while since I looked at that software. I'm pretty sure that Expression Web 4.0 does support .dwt files so that may be the better way to head for you.