XPS 8500, not booting

For a general attempt, you can start with replacing a new coin cell battery. Then performing a CMOS reset using jumper. Refer to you specific model for instructions in the Service Manual.

@Martin van der Graaff,

More info might help, such as the XPS model, what you did 'last'., and did you have a home power event with the PC on or off?

By last, I mean did you install anything or try to, especially a Dell BIOS? If you have a corrupted BIOS, this page should help you, --> https://www.dell.com/support/kbdoc/en-us/000132453/how-to-recover-the-bios-on-a-dell-computer-or-tablet. Suspect this is the case, as you didn't say you saw the Dell Logo at all? I'd also assume F2 doesn't work either?

Re: What are most likely causes

it is possible OP’s Dell pc is showing a Dell logo with a zero progress bar at bottom, which in itself is better than no Dell logo and blank screen (no post).  If we assume this pc is attempting to pass POST (power on test) but getting stalled at the very beginning, that could be due to a piece of firmware failing the test. Most common causes are a failed hard disk drive followed by bad memory stick. 
Try uninstall hdd (and ssd if any) from pc, if there is a discrete gpu uninstall it too, connect monitor to onboard integrated video port, unplug all USB devices, test again. If still 0 progress, remove all ram sticks and install only 1 at time to test. Chino advice of cmos battery and clear are good first aid to apply too.

@Chino de Oro​ thanks for the response. It is an XPS 8500. And old model of more than 10 years old.
I found the manuals.
https://www.dell.com/support/home/en-us/product-support/product/xps-8500/doc

@ispalten​ thanks for your response. I have an XPS 8500, more than 10 years old. 

I earlier created a Recovery boot disk for WIndows 10. Using that I managed to boot and after that I could boot normally without the Recovery disk. Makes me wonder what fixed the problem.

@redxps630​ thanks for your response. See the other answers.

With provided information, there is more to work with.  The XPS 8500 is very old system and can be benefit to change a fresh new CR2032 battery.  The system BIOS settings could be changed when losing power and may have reverted to default settings which in some system, causing non bootable due to storage controller settings.

The XPS 8500 was shipped with mSATA option as a cache drive for spinning HDD to boost performance.  The RST settings may stalled POST if RAID failure is detected.  Remember this is just one of the option and your system may not necessary shipped with it.

As you can see, a specific system model can provide extra information for community to provide valuable input.  Adding the new info I just provide and check your system to see what could likely be the issue.

@Martin van der Graaff​ 

My wife is using an XPS8500... 13 years old by now.

If you intend to keep the XPS (wife's has been updated with an SSD boot, better video card, and newer Wifi capability) I suggest you get some sort of back up solution. I'd suggest Acronis True Image (which she uses) or Macrium Reflect. As PC age, parts may break... disks, video cards, even network adapter or RAM. Even get a UPS as abnormal shutdown can corrupt boot files or others. Having back-ups to restore the complete OS and any other drives you have if it should have makes this a much easier task.

XPS 8500 case can easily upgrade motherboard to XPS 8700/8900 or even XPS 8930 which would enable OP to have official Win 11 support.  any standard microATX board would fit the case which uses standard industry 9 pin FIO connector.

@redxps630 

The 8700 didn't have W11 support? At least not officially. One of the reason's I bough an XPS8940.

Cost wise, any 'replacement' upgrade is better spent on a new PC. Especially considering other components one wants to migrate over... 10+ year old parts are ripe for failure.

I was considering getting my wife an 8960, but will wait until it is really needed. Having added a boot SSD, some more RAM, and another hard drive and using an external USB WiFi 6 adapter sort of brings it up to date for her usage. If she was a serious gamer or needed to 'heavy' work or hobby sort of stuff, it would not be good of course.