While most Windows Live Mail problems are relatively to trace thanks to the friendly error messages, the same is not true of all faults that the application might develop. As such you might opt to take the easy way out and repair the software.
Before proceeding, make sure that Windows Live Mail is completely shut down - exit the software in the usual way, but if this fails right-click the Windows task bar and select Start Task Manager, then find Windows Live Mail (possibly listed as wlmail.exe) and select End Process.
In order to initiate a repair of Windows Live Mail, browse to Start > Control Panel > Programs > Uninstall a program and wait for the list to build. Once this has completed, find Windows Live Essentials in the list and double-click to begin the uninstall/repair wizard.
You will be presented with two options - Remove one of more Windows Live programs or Repair all Windows Live programs. In the first instance you should try the second option, as this should resolve the issue - however bear in mind that that removing the application and then reinstalling it later might be an option. When you select Repair all Windows Live programs the wizard will proceed to repair any problems that it detects.