Perhaps you are experiencing the Intel CPU LPC Clock degradation issue, as I did with a QNAP TS-251 several years ago.
I repaired mine by adding a 100 ohm pull-up resistor, as described in the QNAP forum’s “TS-453 Pro stuck on system booting” thread (any model with an Intel CPU in the Silvermont family is vulnerable to this issue).
Even with the pull-up resistor repair, the problem can eventually reoccur as the LPC clock continues to degrade.
So once I was able to boot it up again, I recovered my files, and promptly sold it as-is, with a full disclosure of the issue and repair. And then I purchased a new NAS.
TLDR; Replace it with a current model, which will have a newer CPU and not be subject to this particular failure.