Splunk licenses do not randomly expire. if you're experiencing issues with your license, you should definitely file a case with Splunk Support.
this is how licensing works:
http://docs.splunk.com/Documentation/Splunk/5.0.2/Admin/HowSplunklicensingworks
this is how license violations specifically work:
http://docs.splunk.com/Documentation/Splunk/5.0.2/Admin/Aboutlicenseviolations
"Violations occur when you exceed the maximum indexing volume allowed for your license. If you exceed your licensed daily volume on any one calendar day, you will get a violation warning. The message persists for 14 days. If you have 5 or more warnings on an Enterprise license or 3 warnings on a Free license in a rolling 30-day period, you are in violation of your license and search will be disabled for the offending pool(s) only -- other pools will remain searchable and be unaffected.
Search capabilities return when you have fewer than 5 (Enterprise) or 3 (Free) warnings in the previous 30 days, or when you apply a temporary reset license (available for Enterprise only). To obtain a reset license, contact your sales rep."
if you're getting locked out of search because you're violating your license, you should be seeing warning messages before this happens.
... View more