I ran in to this problem some time ago whereby for no apparent reason, when I ran iReport the splash screen would appear, but no loading took place, the usual progress bar made no progress.
I have since had a few support calls about this same issue, so I figure I will post the solution here.
The culprit is Java 8 as iReport is not compatible with Java 8 run time - well at least version 3.7.6 is not.
To resolve this issue, simply install the Java 7 JDK (unless already installed) on the machine on which iReport is installed and then configure iReport to use the JDK rather than your local JRE.
2 easy steps:
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1. Download and install JDK7
(http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html)
2. Modify the iReport config file 'ireport.conf' found in the 'etc' folder in the iReport installation directory.
(on a Windows instance; possibly: C:\Program Files\iReport-3.7.6\etc\)
Open the file in Notepad or equivalent plain text editor and find the line:
#jdkhome="/path/to/jdk"
Add a new line below it and replicate the property name (without the hash/"#") with the correct path to your new JDK install, Eg:
jdkhome="C:\Program Files\Java\jdk1.7.0_60"
*File paths are not necessarily correct for your system they are just for examples and are based on a Windows environment, please double check your install paths.
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Here is an unmodified ireport.conf for a 3.7.6 install in case anything goes wrong.
# ${HOME} will be replaced by user home directory according to platform
default_userdir="${HOME}/.${APPNAME}/3.7.6"
default_mac_userdir="${HOME}/Library/Application Support/${APPNAME}/3.7.6"
# options used by the launcher by default, can be overridden by explicit
# command line switches
default_options="--branding ireport -J-Xms256m -J-Xmx512m -J-Dorg.netbeans.ProxyClassLoader.level=1000"
# for development purposes you may wish to append: -J-Dnetbeans.logger.console=true -J-ea
# default location of JDK/JRE, can be overridden by using --jdkhome <dir> switch
#jdkhome="/path/to/jdk"
# clusters' paths separated by path.separator (semicolon on Windows, colon on Unices)
#extra_clusters=