If you are using evaluation version of SCCM, it will expire 180 days later. If your SCCM server expired, you can get it licenced by doing the following:
Type "Configuration Manager Setup" in the search box and run it.
Select "Perform Site Maintenance or reset this site" and click Next
Select Upgrade the evaluation edition to a licensed edition and enter your serial number.
You can check the log "ConfigMgrSetup.log" .
for the process completion.