This problem rears its ugly head at times when you're using ActiveSync with a Windows Mobile device.
What happens is, if the device is syncing anything related to Outlook, an instance of OUTLOOK.EXE runs in the background, and can be seen in Task Manager. For some reason, when you launch Outlook again after this background instance of it is already running, it thinks it doesn't need to bring up the UI.
The solution is to kill all the OUTLOOK.EXE processes from the "processes" tab in Task Manager, then launch Outlook again. The UI should come up after that.
The process goes as follows...
Open Task Manager:
1. Right-click the taskbar and select "Task Manager" or...
2. Hit Ctrl-Alt-Delete and select "Task Manager"
Click on the "Processes" tab in Task Manager.
Click the "Image Name" column to sort the names of the running processes.
Find each OUTLOOK.EXE
Right-click the OUTLOOK.EXE process and select "End Process Tree"
Once all OUTLOOK.EXE's have been stopped, attempt to launch Outlook again.
Hope this helps!