SQL Server Error 5120 occurs when the database engine cannot open the physical database file due to permission issues, file corruption, or incorrect file paths. This error prevents users from accessing MDF and LDF files, leading to database downtime and potential data loss. To resolve SQL Server Error 5120, users can check file permissions, verify account access rights, confirm file locations, and repair damaged database files. In severe corruption cases, using a reliable solution like an SQL Recovery Tool can help restore inaccessible database files, recover tables, and bring the database back to a working state safely and efficiently.