I have a s7210n HP Pavilion Slimline PC, running Windows XP Pro.

Everyting was working wonderfully for 7 months. Then computer stops working. The green LED at the back of the case right beside the power cord comes on when I plug in the power cord. But when I press the power button, there is no noise, and no detectable action at all. The cd tray won't open/close either. The computer won't come on at all, as far as I can tell.

I have cleaned the MB, processor fan and heatsink, I have unplugged all power plugs from power supply and replugged them, I have unplugged and replugged all peripherals, I have checked the power cord, and all other cords and outlets. Everything seems to be in proper order.

Also, there is no reset button on this model, and no additional power button on the back, as there usually is on other PCs.

Does this sound like simply a bad power supply? Or something else? I have no other machine which I can use to check the cpu, power supply, etc. on.

Thanks.

Also, there are no beeps. Nothing but death. I also reseated all cards, etc. Still no good.

Without a known good powersupply you cannot check any other components. I hope for your sake that a generic powersupply is compatible since a proprietary replacement from HP will be expensive. Often in HP systems, particularly their slimline units, they use a proprietary PSU even to the extent of having non-standard motherboard connections. One of the drawbacks to owning a name-brand system.

I used to have a signature but it disappeared and I just couldn't be bothered writing another so please feel free to ingore this.

Didn't you read the manual below at all? There are certain precautions mentioned when you disassembly the unit.

http://h10032.www1.hp.com/ctg/Manua...

i_Xp/VistaUser

Your PS has probably failed. They often don't fail completely, hence in this case the green led still comes on.

There's one thing you could try though.
A common thing that can happen with ram, even ram that worked fine previously, is the ram has, or has developed, a poor connection in it's slot(s).
This usually happens a long time after the ram was installed, but it can happen with new ram, or after moving the computer case from one place to another, and I've had even new modules that needed to have their contacts cleaned.

See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated:
http://www.computing.net/hardware/w...
......

If that doesn't help.....
Check your PS.
See response 4 in this:
http://www.computing.net/hardware/w...

HP no longer uses proprietary oddball main connector and other connector wiring from the PS to the mboard for computers as new as yours, except possibly for server mboards.
For testing puposes, you can connect any PS that has the right connectrors and has same colors of wires on the connectors in the same positions that are on the connectors from your PS, but a standard PS/2 sized PS won't fit in your case.

Brand name system slimline PCs and some smaller towers use smaller microatx sized PSs rather than larger standard PS/2 sized ones. The microatx PS box size was never standardized - there are several possible microatx sizes. Yours is in a rectangular box.
HP doesn't make the PSs themselves - they are supplied to HP by power supply manufacturers. The problem with most microatx sized PSs is they aren't made by major known to be good manufacturers and are not as reliable and long lasting as the average or better regular PS/2 sized PSs.

Whether a PS failing damages something else on the computer while failing is unfortunately random - if you are fortunate, replacing the PS is all you need to do to get the computer working fine again

In this case you can find a replacement PS on the web by searching with the HP part number for the PS - 5188-2755 - for your system.
Beware of used PSs - I wouldn't buy one.
Most listings state a 90 day warranty - you can pay more for a 1 year warranty some places - I recommend the latter in this case.

I recommend you try connecting a used or borrowed PS of any physical size (the capacity of your PS is only 108 watts so that shouldn't be a problem) before you buy a new PS to make sure replacing the PS is all you need to do.