Q: GeoServer doesn't seem to start up because it cannot find data. What is wrong?

A: There are a few things that could be wrong. First of all, GeoServer is probably not finding your data directory. This is where all the configuration files and data lives.
You can point GeoServer at your data directory in two ways: first by editing the web.xml file located under WEB-INF/ and adding the data directory option (it is commented out in the file), the second option is to set an environment variable that points to the data directory. More information on data directories can be found here

Another issue might be that GeoServer isn't acting "quite right" on your operating system. If you have a data directory set up and it still can't find it, check to make sure there are no spaces in the path of the data directory. This usually isn't the case and spaces are usually allowed, but your OS may treat them differently. So, try removing the space.

On Linuz, the issue might be permissions. Make sure that the data directory is accessible by Tomcat. Here is a quote from a user:

"The problem raises using default tomcat5 installation in an
Ubuntu/Debian system. That is because it is very restrictive with
default permissions, so you need to manually edit
/etc/tomcat5.5/policy.d/04webapps.policy file and add the following line
before closing bracket:

permission java.security.AllPermission;

Then all you have to do is restart tomcat and the geosever war will be
automatically deployed. Obviously it will be better to narrow
permissions, but this way I can make it work."