The TLS certificate on the sensor has expired. You just need to generate a new certificate on the sensor.
Use the "tls generate-key" command on the sensor to generate a new certificate:
http://www.cisco.com/en/US/docs/security/ips/6.0/command/reference/crCmds.html#wp504369
You will then just have to update your management station to use the new sensor certificate.
If you need help with that just let me know what management software you are using and what version.