I wish I could re-flash.
The usb port got of the pcb sadly.
But, I still use it sometimes and not every day because it's unstable (shuts down if the flex ribbon moves too much).
Having said that, I didn't open the N900 and still haven't seen what's the problem.
And I went to check what's the problem but the dumb shop worker said the mainboard was dead.
Well, it's not.