Computer wont start up

Hi, that depends... as you can imagine.

Apparently you have a desktop.

When you turn it on, do lights come on, the fans spin... do you see a logo on the screen as it starts up, or only a black screen?

Please describe exactly and precisely what you see and hear when you switch your PC on.

If you do see the normal logo when you power up, are you the also able to access its BIOS/UEFI?
- you haven't said what the make/model/MOBO are.. do you know which key to press to access your BIOS/UEFI?

Other things:
- do you use disk imaging routinely - e.g. Macrium Reflect, Aomei Backupper.. or even Windows 7 Backup and Restore?
- what backups do you have?
- have you made any recent changes? Moved anything? What was the last thing that happened?

Thank you for your reply to part of what I asked.

Would you care to reply to the rest? Doing so will make this quicker and more efficient- for you, and anyone else reading this who may reply. Thank you.

At the Dell Logo Splash Screen, can you press F2 and boot into Setup (Bios)?
Where do you plug your monitor in on your computer? Do you have a Video Card (GPU) lower in the case? Or are you using the Onboard Video out that is closer to the USB ports on the back of your computer? What type of connection are you using? (ie) Display Port? HDMI? DVI? etc?
This may be caused by your Video driver or the Resolution is set too high (ie) Out Of Range. You can restart your computer holding down Both of the Shift keys. Doing this 3 times in a row should bring up the Troubleshooting menu. Here you can choose Advanced. First try a Startup Repair. If this fails, you can also use the System Restore and other options here.

Once you are in your BIOS, can you see your internal disk is present?

How old is your MOBO?

I suggest changing the CMOS battery then setting the BIOS defaults since a low battery level can cause similar issues. Have you done a full power off / on by turning off the PC at the mains switch or unplugging it?