My first thought was overheating caused by dust build up, improper fan 
operation, or poor contact between the CPU and heat sink. But, in 
reading your post, I realized you had already been inside the case 
(seeing the steady green light) and I assume you would have noticed any 
of these overheating issues.

While inside the case, I would also check the connections on all the 
data and power cables. Then I would check to see if any/all cards were 
firmly seated.

If, with the case open, the computer still does not start, more in depth 
diagnosis is warranted. Are there any beep(s) during attempted start up? 
Can you get into the BIOS during attempted start up? Is there any on 
screen information? Depending on the foregoing answers, I would 
disconnect/remove all internal components except motherboard, CPU/heat 
sink/fan combo, RAM, and video card (if any). I would leave the 
keyboard, mouse, and monitor connected. Try starting and notice what 
happens. If the initial POST completes, the problem most likely is in 
one of the disconnected items. If the POST does not complete, and there 
is more than one stick of RAM, I would try each one separately. If the 
POST still does not complete and if there is a video card, I would 
remove it and listen for a single or multiple beeps. If still no POST, 
the problem is likely in the CPU combo or the motherboard. I would 
remove and take apart the CPU combo, clean the components, and reinstall 
using thermal paste between the CPU and heat sink. If still no POST, I 
would try to somehow get a free loaner CPU that is compatible with the 
motherboard and use it temporarily in the CPU combo. If still no POST, I 
would judge that I very likely had a bad motherboard.

BTW Others might strip the computer down to the motherboard and start 
rebuilding it one component at a time until they find the problem. 
Either way the results should be the same.

Good luck.

Tom

mike stanz wrote:
> sounds like processor went bad. I'll bet if you take cover off you'll find
> alot of dirt/dust clogging the fins atop the cooler between the fan and
> processor chip. If thats the case it'll probably cost you $150. to fix this
> pc. Maybe its time to upgrade to a better pc? Good luck.  try staples or
> circuit city for repair.
>
> On 10/6/08, Struan Robertson <[log in to unmask]> wrote:
>   
>> I have a Dell Dimension 5150 which is less than two years old.  It started
>> to balk at starting up a few weeks ago and gradually got worse and now it
>> wont start at all.
>>
>> When I press the start button, a slow flashing light begins just behind the
>> start button while a steady light green light goes on at the back in the
>> region between the back end of the RAM boards and where the leads from the
>> HDD plug into the motherboard - and that's it - nothing..!
>>
>> Can anyone help with a suggestion to what it night be..?
>>
>> regards,
>>
>> Struan Robertson
>>
>>         
>>     
>
>          
>   

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