My signal was perfect at first but now it can't even reach my phone upstairs! Any way to fix it?

I have fiberoptics at home and at first it was great but after a few months of having it the strength has decreased by a LOT! I use WIFI on my PC and also my phone and where at first I could stream things through my phone upstairs with ease it can't even use snapchat or facebook anymore. My PC thats only 4 meters away sometimes has its siganl drop. The sppeds are stupid slow and sometimes even cuts out! 
Why has this happened?!