Failed to check for installd and available applications

I received this error message when I tried to access the 'Add/Remove applications' tool.

"Failed to check for installed and available applications

This is a major failure of your software management system. Please check for broken packages with synaptic, check the file permissions and correctness of the file '/etc/apt/sources.list' and reload the software information with: 'sudo apt-get update' and 'sudo apt-get install -f'."

There is also an error message on the task bar.

"An error occurred. Please run Package Manager from the right-click menu or apt-get in a terminal to see what is wrong. The error message was 'Error:Broken Count>0'. This usually means that your installed package have unmet dependencies."

What should I do?