Appears to be an older desktop build - is that correct? Pavilion perhaps?
Update your post to include full system hardware specs and OS information.
PSU: make, model, wattage, age, conditon (original to build, new, refurbished, used)?
Disk drive(s): make, model, capacity, how full?
Will the desktop boot into Safe Mode? Have you tried installing a new CMOS battery as a matter of elimination?
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Start here:
https://support.hp.com/us-en/computer
Note the Check Warranty Status option. Enter the motherboard's Serial Number.
The results may help find the manual or otherwise what the CR5 led may indicate.
For example:
https://h30434.www3.hp.com/t5/Deskt...ght-on-my-hp-pavilion-is-flashing/td-p/693875
Not a solution per se. The link contains other links that could prove helpful.