Hi back,
Sounds like your mate has disabled (hopefully not deleted) your account which was an admin... this would make the computer boot into guest mode as that is the only option remaining.
By default, in Windows 8 or 8.1 the administrator account sits disabled in the background as a backup and there won't be a password unless you or your mate has set one up BEFORE buggering the main account.
1. When you log in as a guest and open the explorer, can you still see your old account name listed as a folder at C: / Users?
If not (or a bootable back up image) then you are screwed... you can still wipe the system and re-install windows from blank but thats "lots of fun".
If the folder is still there then your old account is just disabled... you didn't tell us what system you have and 8 has different work arounds to 8.1 so thats as far as I can help for now.
2. We need to know the full system i.e. Windows 8.1 professional or Windows 8 enterprise etc...