try replace cmos battery and clear cmos settings. this model is 7 years old.  If problem persists, and power button is iffy to turn on pc, possible failing psu.
I bought this refurbished computer four years ago. Ran fine then started getting sluggish last year so in Sept 2022, had SSD installed. Worked like new. About a month ago, started having intermittent problems when I turn the computer on, it shuts down but not completely. I have to manually power down. Then wait a bit and turn it back on. Last 10 days or so, this became daily. It's so annoying. Today I had to do this twice. I use this for work and really don't want any issues if it doesn't come back on. Yes, files are backed up. 
Dell Inspiron 3668 Desktop need to turn on/off/on again every time