A Lost vCenter Session alarm appears and vCenter Server is unavailable.

An alarm in the Data Director interface states the following:

Lost VCenter Session

In the System Health panel, vCenter Connectivity is associated with a red failure icon.

When you check the vCenter Server system event log, you see the following message:

The transaction log for database 'VIM_VCDB' is full. To find out
why space in the log cannot be reused, see the log_reuse_wait_desc
column in sys.databases
For more information, see Help and Support Center at http://url.

The SQL server that manages the vCenter Server database provides capabilities for recovering the database to any point in time since the last full backup. If no backups exist, the transaction log grows indefinitely or reaches the maximum allowed limit and stops. See VMware Knowledge Base article 1003980 for background information and solution.

Configure regular backups for the vCenter Server database or use simple recovery mode (no point-in-time recovery).