Hi Demonpig,

You can try the steps for no boot mentioned in the link.

https://www.dell.com/support/article/us/en/04/sln292599/resolve-no-power-no-post-no-boot-or-no-video-issues-with-your-dell-computer?lang=en. 

However, check if the system has power do you get fan rotating sound or PSU powering on sound?

Depending on the symptoms you can try no power, post or boot troubleshooting.

https://www.dell.com/support/article/us/en/04/sln284978/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators?lang=en#Three

Diagnostic LEDs for the OptiPlex series (2009 to 2012)

CPU configuration activity or failure; CPU Cache failure

Disconnect all external peripherals and configure PC down to PSU, Motherboard and CPU

LED Pattern(1 2 3 4) LEDPattern StateName StateDescription Suggested Resolution

	Off	Off	System POST complete — LED time-out has occurred	If the LEDs came on and then went off, look at troubleshooting the display and the operating system. If the LEDs never came on at any point then look into a power issue.
	4	RCM	BIOS checksum failure — Recovery mode	Reset the BIOS and clear the NVRAM If this occurred after windows updates, check your startup wsn't corrupted.
	3	CPU	CPU configuration activity or failure; CPU Cache failure	Disconnect all external peripherals and configure PC down to PSU, Motherboard and CPU If fault continues contact your technical support If fault stops, plug disconnected parts back one at a time until fault re-occurs. Contact your technical support to take this further.
	34	MEM	Memory subsystem configuration activity or failure	Remove all memory from PC, does the LED code change? If it doesn't contact your technical support If it does add the memory modules back one at a time until the fault re-occurs, test that module in another slot and contact your technical support
	2	PCI	PCI device configuration or failure	Remove all external peripherals and all internal PCI devices If fault continues contact your technical support If fault stops, add parts back one at a time until fault re-occurs and contact your technical support
	2 4	VID	Video subsystem configuration or failure	Ensure the video cable is connected to the correct port Reseat any add in graphics cards If fault continues, remove any add in graphics cards and test with known good monitor and video cable Contact your technical support once you know which part the fault follows
	23	STO	Storage device configuration or failure	Reseat the cables from the HDDs to the motherboard If fault continues try substituting a known good HDD into the system and contact your technical support
	234	USB	USB subsystem configuration activity or failure	Remove all external peripherals and any internal CRUs If LEDs change, add parts back one at a time until fault re-occurs and contact your technical support If fault remains the same with parts removed, contact your technical support
	1		No memory detected	Reseat all memory modules If fault continues, try the memory in different slots and contact your technical support
	1 4	MBF	Fatal system board failure detected	Contact your technical support
	1 3		Memory is detected, configuration or compatibility error	Reset the BIOS and clear the NVRAM and any previous errors Remove all memory from PC, (This includes any Graphics cards.) does the LED code change? If it doesn't contact your technical support If it does add the parts back one at a time until the fault re-occurs, test that part in another slot and contact your technical support
	1 34	PRV	Indicates system activity preceding video initialization	Remove all external peripherals and any internal CRUs If LEDs change, add parts back one at a time until fault re-occurs and contact your technical support If fault remains the same with parts removed, contact your technical support
	12	CFG	System resource configuration	Reset the BIOS and clear the NVRAM and any previous errors Remove all external peripherals If LEDs change, add parts back one at a time until fault re-occurs and contact your technical support If fault remains the same with parts removed, contact your technical support
	12 4		Reserved	Reserved
	123	POV	POST complete — Onscreen error message	Troubleshoot as per the error message onscreen
	1234	STD	POST complete — OS hand-off complete	The Power On Self Test has passed all checks Troubleshoot the HDD and the Operating System