It's mainly related to the first three reasons in my answer! So, you should make sure that
1.the DB server is up and running,
2. the SQL Server default port is opened and you can reach the DB server from the App server,
3. all SQL services are started!
4. Finally, you should check with the DB and network/infra team to ensure that there are no changes have been recently applied especially for the IP, default port, and server name of the DB server!
Good Luck!