No. 1.2.3. here is not a step, but each individual question.
1. Do not log in to the linked database
sqlplus /nolog
2. ORACLE view and modify the maximum number of connections
alter system set processes=1000 scope = spfile;
alter system set session=1105 scope = spfile;
alter system set DISPATCHERS = '(protocol=tcp)(dispatchers=3)';
alter system set pga_aggregate_limit=1024M
alter system set pga_aggregate_target=4096M
3.hour-01034, now-27101, now-00093 ：
Find file D:/app/orcl/admin/orcl/pfile/init.ora.31201795943
Added (modified): shared_pool_size=10485760
Then execute connect sys / as sysdba under cmd ,
startup force pfile='D:/app/orcl/admin/orcl/pfile/init.ora.31201795943'
You can see "The database has been loaded" and "The database has been opened";
4.ora-12518: error handling
Problem description: When looking at the service, the oracle database has stopped
a. Click to start
b.dos window, sqlplus /nolog, then conn sys /as sysdba, enter password, show connected routine
c.输入 startup force pfile='D:/app/orcl/admin/orcl/pfile/init.ora.31201795943'
d. The database restarts successfully and can be connected normally