7050 Micro doesn't POST after being stored for months

Pressing power button x 30s is a way to clear CMOs in later Dell models that don’t have clear CMOs jumper on mobo.

7050 M mobo has a jumper so I would try that. https://www.bhphotovideo.com/lit_files/373141.pdf

p.21

(clear RTC by power button does not work in 7050 model)

Replacing battery does not always do the trick. PC in storage often has CMOs corruption despite 3V on CMOs battery. It is supposed to be non volatile memory but in reality it can suffer lapse when not connected to power for some time 

• Carefully move the plastic jumper cap from its default pins to the clear CMOS pins (e.g., from pins 1-2 to pins 2-3).
• Wait 10 to 30 seconds for the CMOS to be cleared.
• Move the jumper cap back to its original position (e.g., pins 1-2). 

• Dell G5 5000 MT
• Dell Precision 3440 SFF
• Dell Precision 3640 MT
• Inspiron 3680 SFF
• Inspiron 3880 MT
• Inspiron 3881 MT
• Inspiron 3891
• OptiPlex 21-3280 All-in-One
• OptiPlex 24-5480 All-in-One
• OptiPlex 24-7480 All-in-One
• OptiPlex 27-7780 All-in-One
• OptiPlex 3080 Micro
• OptiPlex 3080 SFF
• OptiPlex 3080 Tower
• OptiPlex 5080 Micro
• OptiPlex 5080 SFF
• OptiPlex 5080 Tower
• OptiPlex 5090
• OptiPlex 7080 Micro
• OptiPlex 7080 SFF
• OptiPlex 7080 Tower
• OptiPlex 7090
• Vostro 3090 MT
• Vostro 3681 SFF
• Vostro 3690
• Vostro 3881 MT
• Vostro 3888 MT
• Vostro 3890
• Vostro 5880 MT
• Dell desktop computers launched prior to this document have a motherboard jumper-based reset function. If your product was shipped prior to April 2020 and is not listed above, then the computer most likely has a jumper-based reset. See your product service manual for more details.

@redxps630​ thanks for your answer. I found the plastic cap already on the "clear CMOS" jumpers when opening the lid. Anyway I removed it, plugged again, left it there then re-plugged the power cable but I still get the 5W2Y led. I tried also the "Service mode" jumpers (right couple according to the instructions pdf), with no avail.

One thing I'm noticing is that the computer keeps turning on by itself even if I shut it down long pressing the power button. 

Re: plastic cap already on the "clear CMOS" jumpers

it is on default or normal position, which does not clear CMOs. You need to remove it and put the jumper on the other two pins to clear CMOs.  After 10 sec move back to original position.

if jumper were on clear CMOs position and you try to power on pc, pc would not start.

@redxps630​ yes sorry, I misread the scheme on the pdf.

So, I plugged in the power cable, the computer turned on by itself, I put the jumper on the clear CMOS pins (1-2), the computer shuts down, I waited 10-12 seconds, put the jumper back to the center, default pins 3-4. As soon as I removed the jumper from pins 1-2, the computer turns back on with the 5W2Y led blinking. Did I do something wrong here? 

I usually unplug the pc. Move jumper to CLR CMOs position for 15 sec, then move back to default position, then plug in power cord again to test.  If this does not make a difference, and you have already replaced CMOs battery with a new one, then I don’t know what else to suggest.

@redxps630​ I followed this procedure too, multiple time, with no progress whatsoever so I will throw away this computer and never get a Dell ever again. I thank you anyway for your support.