My 4s not connecting to WiFi unless im next to the box?

I got my iPhone passed on from my mum and the Wi-Fi on it was working fine. its a white 16 GB iPhone 4s Today I was on my phone from my room and it wouldn't connect to Wi-Fi, I went right next to the box and that's when it worked. the same problem used to happen when mum had it but it was fixed after I erased everything and started as a new iPhone. now my 4s wouldn't connect to the Wi-Fi unless I'm right next to it. how can I fix this? I upgraded it to IOS 7.1.2 'but that did not fix anything. I need help ASAP