The common cause of Lync FE servers service to hang is related to non self-assigned certificates found on the Trusted Root Certificate store. Lync servers does use HTTP/s to communicate each other and the issue mentioned will break this behavior. Until such certs removed, FE service wont come back to its health state.
Step # 1
Identify the certs causing this issue, run below PS from problem Lync FE Server.
Step # 2
Delete the problem certificates from Trusted root store, it is easy to identify these certificates as
"Issued From" and "Issues To" wont match. Check to make sure these certs are not getting pushed via GPO if so, you will need to make sure such GPO is cleared out.
Step # 3
Reboot FE server and make sure FE service comes up. Check event logs to make sure related event logs are cleared.
Principal Systems Engineer