Thank you.Probably, you can't. If it stopped spinning then either the fan has died, and since you say you don't know when it died it's likely it died from a lack of maintenance. Dust and dirt, when ignored, tend to kill fans and motors. Or, given the age of the system, it could very well be that the fan controller on the motherboard has failed which wouldn't be surprising since that motherboard is about 14 years old. Either way, "getting it to turn back on" is almost certainly not going to happen. If it was working, it would be turning or at least trying to turn. Any fan that's been running for 14 years has certainly outlived it's expected lifespan and should be replaced. No question.
Most likely the rear fan is either an 80mm or 92mm but you will need to measure side to side from one edge to the other. Report back with that measurement and I will show you fans that you can use as replacement options but I need to know what country you are in or where you can order parts from. Or, if you have to buy from local shops I can tell you what you need to look for.
Thanks.All fans turn if you blow air on them. Even if they aren't plugged into power.
But, you can always try this as well if you are handy enough and feel confident to attempt it. It's possible that there is simply too much gunk built up inside the fan bearing and no lubrication left. I wouldn't count on it but if replacing it is for some reason (Fans like this can be bought for like 5-10 bucks, sometimes less) then it might be worth at least trying.
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Keep ‘Em Spinning: How To Lubricate PC Fans - Overclockers
Sleeve bearings fans have a well-deserved reputation of wearing out quickly. But they're inexpensive, and not necessarily cheaply made. They can perform very well indeed. We want to use them on our heatsinks, but we worry that they will fail and leave our CPU's overheating. Wouldn't you like to...www.overclockers.com
If the side panel is removed and the CPU cooler fan is still working, then you can probably get by without it. Seems like a lot of extra effort for a five dollar part though.