Error establishing connection to SQL Server

from SSMS to a previously local SQL Server I was able to access it, but I can no longer access it

Please tell me how to deal with it

A network-related or instance-specific error occurred while establishing a connection to SQL Server.Server not found or inaccessible.Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error locating the specified server or instance) (.Net SqlClient Data Provider)