How to fix gears?

I've had this bike kindly lent to me 2 days ago and its in perfect condition. Heard a humming noise yesterday and this morning but it stopped after a few seconds but this afternoon the noise wont stop till i go upto gear 7 and when i bring it back down any lower gear the noise comes back and doesnt stop. I cant have gear more than 1 because im trying to get fit after many healthn problems inc heart transplant and tension has hired so I can't use it even though it's saying level 1. I cant see how ive broken gears because ive only slowly peddled a few miles but can any1 help me to fix this problem please. It's a Reebock RB1000. Many thanks Sam x