hi,

anyone have this issue? its reset user to default (admin, no password).

No, perhaps you or the routeros itself restored a backup with no password?

ive had this happen on a singel router. i think it was caused by bad flash - cuz after a reboot it would be back to normal.

thats happene once with me before when the harddisk disconncet from the system. after reboot the system and conncet the hard evey thing back to normal.