Usually onboard videos may need turning off to allow cards to operate! (on older machines it was a jumper. Pci cards shold take precedence but it could be a driver problem.
I presume no go in safe mode?
Is there any beeps (3 indicates at startup video problem! (ie cant find video card or monitor)
However no screen display at all on start up could even indicate processor problem or memory as these have to work to get anything on screen!
BOL
M

Thanks for your input mjdodd. 1) no jumpers on this mainboard. 2) I currently know of no way to get into safe mode without any display, and long before Windowsxp is started, the bios setup program cannot be seen either as no display is ever available. 3) Problem sequence: a)Some register cleanup application was downloaded and allowed to run for 45 minutes. b) application completed and claimed to have found 558 problems, offered to fix them for 50 bucks. c)application was removed successfully from computer, d) computer was allowed to run all night long (laziness), e) next AM computer was properly shut down and rebooted and black monitor, no visibility, gripped this computer. My buddy asked me for help at this point. I am here because he has no use of his computer currently. 4) Thankyou for confirming my belief that pci card should have taken precedence. 5)This computer has never had any speakers, other than the little one inside the tower - NO BEEPS at all under any attempted reboot we have tried. 6) If we assume a processor or memory problem it would be quite a coincidence for the above sequence to "cause" memory or processor failure, but I suppose it is possible. Problem occurred instantly after this Problem Sequence. Could a power supply go lame after being run all night, and would a 'weak power supply' be too weak to support a monitor? We have not swapped that out yet due to the expense, but I am suspicious about the power supply but I have no rationale for being suspicious. Any ideas re" the power supply? Thanks again for your input. I am going on trial and error and I have no clue.

letting you computer run over night is not your problem ,computers can run 24/7 for years!
take the case off the computer ,turn it on any activity at all like fans lights on motherboard ,a power supply can go at any time if it failing ,hows the dust situation inside the case

Thanks caperjack,
Dust was horrible, beyond the pale. First thing my buddy did (after onset of black screen), was vacuum all the dust out. Computer guts are quite clean now.

Is there a way to test the power supply? Or is the only really good way to swap it out?

Thanks again for your help.

dust is a killer of electrical components ,cause some them to over heat and gradually fail ! you can test it with a volt meter or a Power supply test ,swap is your best way if you don't have either.
when you turn on the computer does anything happen ,like fans turn on or keyboard light's turn on ,anything

hey dude, you runing a Desktop right? Take out all the components one by one (including the CPU), clean them with a clean lint free cloth, put them back. make sure the HDD's are booting up by listening to them, they should spinn, if they don't, it's your PSU, if they do, look for Mobo lights and keyboard lights, all should light up with a working PSU and good Mobo!

Fan spins up normally. Disk drive spins up normally (good sound). I have not looked to be certain every light is on, but I am pretty sure all lights light up normally. I really think lights out would have caught my attention. (but I will double check this. But NO BEEPs of any kind which seems a little strange. Monitor, and vid card are swap-ins and known to be good.

Thanks for comments fearless. I will try that and get back to you.

I cant remember the beeps list but google will have it. No beeps I think ois good, any other beeps indicate probs.
I totally agree withthe dust etc killers it can be.
PSU can be tersted with a voltmeyter you will be looking for 5v 12v _5v _12v a,d in some cases 3.5v.
However if you do not see the name of the Bios OS on start up then the bios is ot starting and that indicates that therer is a problem of no volts or a cooked mother board or a bios failure!
I agree computer can run for ever but they can get too hot through dust etc and this can have one or more problems.
most modern PCs have heat cut outs in the processor to avoid damaging it by overheat,
the PSU does not normally have heat cut out!
the moboard has a number of other peripheral chips like the cpu support chips, bus directors, disk drive and bios etc. these can all or singularly fail by overheating; as can solder joints on the tiny rtracks on the motherboard;

If you really cant get anything on the monitor and the video board and monitor are OK then as above try areplacement PSU but beyond that I suspect you need professionapo help as it seems more likely a motherborad or chip failure.
M