Terraform state lock -- stuck lock after crashed apply

Terraform refuses to plan or apply with 'Error acquiring the state lock'. A previous terraform apply crashed or was interrupted, leaving the state locked.

First verify no other process is actually running terraform. Then: terraform force-unlock LOCK_ID. The lock ID is shown in the error message. For S3 backend, the lock is stored in DynamoDB. Never force-unlock while another apply is running -- this can corrupt state. If state is corrupted: terraform state pull > backup.json, fix manually, terraform state push backup.json.

Terraform locks state to prevent concurrent modifications. If the process crashes before releasing the lock, it stays locked until manually cleared.