Lexard:
Lots of thanks to everyone who posted in this topic, however, it's pretty weird that it's broken because it hasn't fell even once or gone through any break downs.
Animals (people, etc) and machines can fail (at some time or any time), even when 'new'. They can be produced/created with 'faults' or something wrong.
With servo motors, there may even be cases where soldering to internal terminals might not have been done properly during production/assembly....leading to disconnected wiring. Components can be defective. So it's often advantageous to have spares ---- for backup and testing. Sometimes, there can be lucky breaks if you identify the issue and manage to fix it.
If you have other units of the same kind that work, and this one is the only one that isn't working at the moment, then it's obviously failed.
Lexard:
how do I see if the wires are damaged?
That would require disassembling ---- eg. some youtube videos for SG90. Then maybe using a multimeter to test for electrical continuity between internal terminals and external terminals (of wires).
Lexard:
That's why I'm still indecisive whether it really is damaged completely.
You probably mean you're uncertain about whether not it is repairable (or too much effort to repair), right? Only time will tell. Disassembly, observations and testing.