How to troubleshoot my sap system when it was down? What are the things I have to check?

A Basic Technical Troubleshooting:
Perform initial analysis.
Identify if something has changed (software or hardware installation, scripts, profile parameters, database structure).
System (hardware) is down:
- Check console messages.
- Reboot and keep important system log files.
- Check hardware components.
- Analyze with diagnosis tools.
- Escalation procedure: call hardware support.
- Check main log files.
- Analyze with diagnostic tools.
- Start up/shut down log files.
- Check operating system log files.
- Identify other problems (startup/shutdown, disks and operating systems, database, operational R/3, performance).
Startup and shutdown problems (R/3 and application servers):
1. Check processes or services at operating system level.
2. Check whether database is up.
3. Check connection with database (i.e., tnsping, R3trans). 
4. Check whether file systems are full. 
5. Check for network problems in the server network. 
6. Check backup log for problems after backup. 
7. Check for problems after system crash. 
8. Check for problems with PDC file server. 
9. Check files permissions and authorizations. 
10. Manually start up or shut down database and then the application server. 
11. Check hosts and services files.
Operating system problems:
1. Check operating system log and event viewers.
2. Check disks and file systems.
3. Check directories and permissions.
4. Check path names.
5. Check security.
Database problems:
- Starting and stopping database
- Refer to startup problems.
- Check database specific error log file.
- Database storage status
- Check database error numbers.
- Check database alerts.
R/3 application operational problems:
1. Check whether all users are affected.
2. Check whether there is no work at all, or whether work is just limited. 
3. Check whether application servers are available (SM51). Check with standard monitoring transactions: SM50, SM04, SM12, SM13, ST22, SM37, and SP01. 
4. Check trace files on work directory (/usr/sap/<SID>/<instance>/work). 
5. Check SAP systems alerts. 
6. Analyze database problems.
Specific operational problems:-
Problems with printing:
A. Check network.
B. Check SP01/SPAD spool request logs.
C. Check system log.
Problems with batch input:
A. Check directories and files.
B. Check SM35 logs.
C. Check system log.
Problems with lock entries:
A. Check lock entries.
B. Check update process.
Problems with update process:
A. Check update process.
B. Check database.
C. Check network connections.
4. Problems with enqueue:
A. Check database.
B. Check instance parameters.
C. Check operating system processes.
5. Problems with message server:
A. Check processes.
B. Check network.
C. Check log files at work directory.
6. Problems with background processing:
A. Check job log.
B. Check background system consistency.
C. Check file systems.
D. Check system log.
7. Performance problems:
1. Perform workload analysis.
2. Perform debugging.
3. Perform runtime analysis.
4. Perform SQL trace.
5. Check for operating system problems.
6. Check disks for input/output bottlenecks.
Resolution process:
1. Search or SAPnet notes for hints, solutions, or workarounds. 
2. Escalation: enter message on SAP hotline or SAPnet. 
3. Enter problem on hardware partner hotline. 
4. Follow problem actions and SAP indications. 
5. Apply patches and corrections; test and verify results.