OptiPlex GX150 series. Diskette Drive 1 Seek Failure. Primary Hard drive 1 Not found. Secondary Hard drive 1 not found.

If both of the hard drives are on the same data cable/disk drive controller it is unlikely that both the have failed. Reseat the data cable connectors on the hard drives and at the motherboard.
Reboot you computer and hopefully it will boot up.
Check the BIOS configuration to see if the BIOS detects the hard drives.
If not the disk drive controller on the motherboard may be faulty.

maybe the floppy disk drive is defective or maybe the IDE cable is deffective, or maybe the cable on the floppy drive is not properly inserted , try to replace IDE cable of the floppy drive if still the symptoms happens replace the floppy drive

This message means that the BIOS is unable to find a drive with an Operating System to boot from.

Typically this means that the IDE cable that connects from the motherboard to the Hard Drive is not connected

It may also mean that there is no power cable plugged into the Hard Drive.

Check those first.

This means your hard drive has either failed or is disconnected.

Given the age of the computer it's most likely the drive has failed.

Let me know if there is important data saved on the computer. Otherwise, replace the drive and re-install windows and it should work again.

Crankstart

Hmmmm That is strange.
If all three of these error messages started at the same time it probably means that something has seriously gone wrong with the Input Output controller(s) on your motherboard.

Have you moved house recently? Sometimes cables can fall off inside the computer when you move them although it is unlikely that your floppy and hard drive cables would both fall off.

When the computer starts go into the bios settings the button you need to press normally flashes up on the screen. Like F2 or Del or something like that. If the date and time is wrong the battery on *********** board may have gone flat. Not a big deal to change that.

Otherwise you may be stuck.

If there is a diskette in the diskette drive remove it. It's a bad diskette.
If no diskette in the drive the drive may be bad. Till you can replace it disable the diskette drive in BIOS.
access bios by F2 at boot up and disable the Diskette drive (Floppy drive)

Check your BIOS setting to see if it detects the hard drive (Primary disk 0). If it can't detect the hard drive then the hard drive maybe faulty. Open the case unpluge the IDE cable from the hard drive and reconnect it again, also unplug the IDE cable at the motherboard and reconnect it again including the power connector on the hard drive.
There maybe an option to test the hard drive in the BIOS, if so use it to test the hard drive.
If the hard drive is faulty, you need to replace it and install Windows, the various device drivers and your programs to get a functioning computer again.
You might consider buying a new computer (it will be faster & better than the old Dell) and you have your new monitor to go with it.

Sounds like you have a bad/disconnected Hard Drive and the system is looking for a second boot device (ie: Floppy with a disk) or your primary IDE channel on your motherboard is bad

Boot into setup by pressing F2 or F10 depending on your bios and disable the floppy drive setting.

make sure they are set to main hard : Master on one cable
2nd : slave on the same cable as the first hard drive
3rd : as master or cable select on the second cable
windows should load then