Stuck in bootloop.

Had the same problem with this one beep and then a reboot. Bought a second NAS540, cause I thought it was a hardware issue. 
Installed the HDs in the second one, restored the 6 months old config from my first NAS. After 1-2 fails in the WebUI, it runned one time till the same problem came again! 
Thats what I figured out / my thoughts:
It wasn't a hardware issue as well, I think its more a defect in the software configuration file.

To make it run: You have to catch the point, when the NAS is ready to reset, with the reset button on the rear, that the NAS can restore the defaults (no worries, the filesystems will be recognized after that). Catching the point is tricky, but you know when you got it when the NASs working LED blinks and a reboot caused of the effect from the defaults restoring...

In my case I could made it run again on my NAS, not on the 2nd, bought one. While I figured this out, I think I crashed the 2nd NAS one by trying (pulled out an HD while booting). When I start it up now, it beeps all the time.

try it with the rescue stick of @Mijzelf ?