some one has changed my administator now i carnt do any thing on my own com

my computer broke and a friend of a friend said they will fix it for me for a small some of money (as i am a single mum and dont have a lot of money comein in) he fixed the computer but changed my administator and now i carnt change the date and time as this is wrong and i carnt do anything on my own computer it is realy stessin me out as the person who fixed it n changed it has disserpeard from the face of the earth and im stuck as im not very good with computers can someone please help i would be verrrry greatfull best regards to every one xx