Dell desktop recovery from no boot

We have a Dell Dimension 9200 with 4GB of RAM and a nVidia GeForce 210 card.
It is lightly used for little more than occasional browsing, YouTube viewing and watching movies.
Now, it's about 4 years old but performs well.

The other day, however, it failed to boot.
Well, actually, it froze during the boot. On the Dell splash page, the progress bar that normally rapidly races to the end, this time stopped about 40% of the way across.
And it did this time after time. Never better, never worse.

My first thoughts were that some part or other had died.
Then I came across this thread which discusses something similar but unplugging the computer allowed it to boot.
However, this wasn't seen as a solution because it hung again on the next boot necessitating unplug/plug once again.

But, I tried it making sure when the power cable was unplugged that I held the stop/start button pressed for 10-20 seconds.
After this, sure enough, it booted as per normal.

However, a subsequent shutdown and boot gave the same freeze at the same place so this wasn't a permanent solution just as the OP in the linked thread reported.

Incidentally, when the freeze occurs diagnostic lights 1, 2 and 3 are green but 4 doesn't show at all.
Unfortunately, according to the Dell manual for this box, this signifies "Another failure has occurred". Well, that doesn't really narrow things down a lot.
The details, however, does suggest loose HDD or CD-Drive cables as one possibility.
Tomorrow, I'll open up this machine and make sure everything, cables and cards, are seated firmly.

Nevertheless, unplugging the power cable and, importantly, holding the shutdown button pressed for 10-20 seconds, has invariably given rise to a normal boot for me.

Additionally, all this suggests that no fundamental part of the machine is damaged which means no major expenditure.