Why did my router's OS change and help to fix?

Ok so I'm gone for the weekend. Come back and the internet is down. About an hour later it comes back on and I can't go on Xbox LIVE because the MTU is to low. This was odd because I had this problem when I first got my internet and fixed it by going into my netgear routers configuration by using the default password and changed it. Now I go onto my computer to change it, type in my ip, and now MikroTik shows up!!! For one I or nobody in the house installed this OS and now it's there. Worse even I did not know the password, which is now frustrating me because I pay for my Xbox LIVE service and I am not able to play because of an unexpected magical change of my routers OS. Anyways, I search around try the default password for Mikro Tik (admin and password left blank) and it says those are invalid. To me this makes no sense and it is extremely frustrating, care to enlighten me on the subject and how to log into my settings?Why did my router's OS change and help to fix?
try doing a hard reset on the routerWhy did my router's OS change and help to fix?
If you have wireless, it is possible someone is piggybacking and changed your OS via wireless, but i doubt it. A hard reset is probably the way to go.