That would take a computer forensic team. The most common issue I encounter is the password and logins are reused. For example many folk have they name or password the same across many sites.

There are many articles about that and why it's such a bad idea. One leak by a retailer or an infection on a PC that you have the browser remember passwords and it's game over.

Given the password reuse is most common I've found that to be the cause in about half the cases.

https://www.google.com/search?hl=en&as_q=&as_epq=HTTP+login+Detect+abnormal+logins+at finds priors on this but again you want to know what happened.

My bet is your PC was infected, you left your router setup to allow WiFi admin logins or you use the same name and/or password across many sites.