The emails are usually moved to Outbox folder because of problems with internet connection. It could be also caused by an incorrect configuration, but if you were able to send emails before, it shouldn't be the case. You have to try the following steps:
1. Check if you are connected to the network with internet access
2. Check if you are not working in offline mode. You can enable or disable offline mode in Send / Receive section by clicking on Work Offline
. When offline mode is enabled, you can see the notification in the right bottom corner.
3. Click on Send / Receive All Folders button
in Send / Receive section.
4. Restart Outlook - close all the Outlook windows and run it again.