Lync 2010, running on Server 2008 R2, been running for years and all of a sudden the front end service will no start.

The following Error

Windows could not start the Lync Server Front-End on Local Computer, For more information review the system event log. If this is a non-Microsoft Service, contact your service vendor and refer to service-specific code -2146762495

Results from start-cswindowsservice

  4/27/2015 10:06:47 AM   Completed with errors

         Host name: nohlyncp1.nebrortho.local   4/27/2015 10:06:47 AM    

         Service start complete "Lync Server Master Replicator Agent (MASTER)".   4/27/2015 10:06:47 AM    

         Service start complete "Lync Server Replica Replicator Agent (REPLICA)".   4/27/2015 10:06:47 AM    

         Service start "Lync Server Front-End (RTCSRV)".   4/27/2015 10:06:47 AM    

         Error: Service "Lync Server Front-End (RTCSRV)" failed to start within the expected timeframe: System error..   4/27/2015 10:06:48 AM   Error

         Service start "Lync Server IM Conferencing (RTCIMMCU)".   4/27/2015 10:06:48 AM    

         Error: Service "Lync Server IM Conferencing (RTCIMMCU)" failed to start within the expected timeframe: System error..   4/27/2015 10:06:49 AM   Error

         Service start "Lync Server Web Conferencing (RTCDATAMCU)".   4/27/2015 10:06:49 AM    

         Error: Service "Lync Server Web Conferencing (RTCDATAMCU)" failed to start within the expected timeframe: System error..   4/27/2015 10:06:52 AM   Error

         Service start "Lync Server Audio/Video Conferencing (RTCAVMCU)".   4/27/2015 10:06:52 AM    

         Error: Service "Lync Server Audio/Video Conferencing (RTCAVMCU)" failed to start within the expected timeframe: System error..   4/27/2015 10:06:53 AM   Error

         Service start "Lync Server Application Sharing (RTCASMCU)".   4/27/2015 10:06:53 AM    

         Error: Service "Lync Server Application Sharing (RTCASMCU)" failed to start within the expected timeframe: System error..   4/27/2015 10:06:55 AM   Error

         Service start "Lync Server Web Conferencing Compatibility (RTCMEETINGMCU)".   4/27/2015 10:06:55 AM    

         Error: Service "Lync Server Web Conferencing Compatibility (RTCMEETINGMCU)" failed to start within the expected timeframe: System error..   4/27/2015 10:06:58 AM   Error

         Service start complete "Lync Server File Transfer Agent (FTA)".   4/27/2015 10:06:58 AM    

     Command Status: Command processing is complete.   4/27/2015 10:06:58 AM    

     Command Status: Command run is complete.

Any help or thoughts would be greatly appreciated.

Can you double check that your certificates haven't expired (as well as any intermediate or root cert authority you may have): http://blog.icewolf.ch/archive/2013/01/06/lync-server-front-end-service-cant-start-because-of-expired.aspx

Also check the Lync event log for any other errors.