If you work with web sites, DNS, etc. and have to make regular DNS changes, it can often take a while for DNS changes to propagate. So you can either wait the 12 – 24 that it takes for your computer to refresh, or you can try and force a refresh by clearing your local DNS cache. It doesn’t always work, mind you.
If you are using Mac OS X, you can try:
dscacheutil -flushcache
Or if you use Windows you can try:
winipcfg /flushdns
You may need to restart your browser or which ever application you are trying to access the site with.