Unable to connect to specific instance of Oracle RAC
If your TNS entry is incorrect for your current configuration, for example you specify (SERVER = SHARED) in your connection descriptor, but you have not configured shared servers, you will receive this error.

But since you said you had not changed anything and this error had occurred suddenly, I am going to assume your configuration is correct. In that case, the database instance may have reached the limit for the processes it can start. You should check the alert log for ORA-00020 errors, and query the highest ever (since the last restart) process utilization of your instance:

select max_utilization, limit_value from gv$resource_limit where inst_id = 1 and resource_name = 'processes';


If you find ORA-00020 errors, or max_utilization equals to limit_value in the output of the above query, the instance reached the limit at some point, and you may need to increase the value of the processes parameter or investigate the reason for having such number of processes.