If you cannot log graphically into your machine (first 3 common problems), there is a trick to boot your system into "single-user" mode. This mode does not provide networking, graphics, or logging in as other regular users, but can connect to a text-based interface as root. This method will only work if a GRUB (<u>'''Gr'''</u>and '''<u>U</u>'''nified <u>'''B'''</u>oot-loader) password has not been set, or that you haven't forgotten the GRUB password.