The First Question Of Troubleshooting: what changed between the time things
worked and the time they didn't?
The Second Question of Windows Troubleshooting: what is the malware/virus
status of the machine? If you think it is clean, what programs (and
versions) did you use to determine this?
And if I may add to Malke's excellent "first question," a common response is
"nothing has changed."
That is *always* the wrong answre. Something always has changed. If nothing
had changed, nothing could have gone wrong. The job of the troubleshooter is
to find out what has changed.