my Macbook pro won't connect to thunderbolt

I have bought my Macbook Pro from my sister in law, with the thunderbolt screen and connectors too. now my sister in law has never had any problems with this but since Yosemite it seems to be playing up. at first it would connect and go onto the bigger screen but would black out etc. lately though I have gone to connect it up and the screen won't come up on thunderbolt at all, just stays black. The charger port works fine, but when i put the thunderbolt cable inside the port nothing happens to either screens. its as if its not registering at all? not sure what to do, as contacting apple will cost me £25 and money is short, any advice would be amazing please, ps i am still getting use to this computer so if theres any walk throughs please make it as simple as possible.