Most people will tell you to check your proxy settings or delete and re-add artifacts, but I will stay away from that and give another suggestion in case that doesn't turn out to be your problem. It could be your mirror settings.
If you use maven at the office then there's a good chance maven is configured to look for your company's internal maven repository. If you're doing some work from home and you are not connected to the network this could be the problem. An obvious solution might be VPN to the office to get visibility to this repo. Another way around this is to add another mirror site to your /User/.m2/settings.xml file so if it fails to find it on your office network it will try public repo.
<mirror>
<id>Central</id>
<url>http://repo1.maven.org/maven2</url>
<mirrorOf>central</mirrorOf>
<!-- United States, St. Louis-->
</mirror>
For other maven repositories take a look here: http://docs.codehaus.org/display/MAVENUSER/Mirrors+Repositories