This is almost always a licensing / activation issue.
On your core, go to Start > LANDesk > Core Server Activation. Click Activate, let it run, see if this solves the issue.
If not, click on the license tab, did your license expire?
If step one does not solve the issue, you will need to call LANDesk support, sometimes this is just an error on their side that they need to fix, or it could be an expiration issue on yours, either way, they can help.