Wifi signal has got weaker

All of a sudden my wifi signal is not travelling so far as hitherto. My PC and router is downstairs. Up to now I have been able to use my wifi on ipad and iphone upstairs but now the signal disappears when I go upstairs. I haven't moved anything or changed anything. As this happens with my ipad and my iphone I think it's definitely the signal and not the devices. Has anyone any suggestions on how I could solve this. Thanks in advance! By the way I'm with telenet.