Dell Inspiron 15 series 5000 won't start up

Registration date
Friday October 16, 2015
Last seen
October 16, 2015
Hello there! My dell inspiron is not starting correctly, it happened weeks ago and I don't know how to fix it. When I open my laptop it either want me to restart it (where nothing will change) or go to advanced setting, I want to use and try all things but it says there that only administrators can do it, which I can not do because my sister was the last one who used it, how can i change my account from guest to administrator? Please help me