My internet is extremely slow and sometimes not working on any device in my household, Help?

[Phil]

My internet (iiNet Broadband, Australian) is very slow on every device in my household, brother tries Youtube, doesn't work. Mum tries regular internet, very slow. We've tried restarting it a couple times but does nothing. On my PC, for example, the internet is EXTREMELY slow. Can anybody help as this is not simply deleting cache and restarting the computer? If so, thank you!
After reading the first answer, I'll say that the internet has worked perfectly before, and has only started to work terribly since two days ago.
After reading the first answer, I'll say that the internet has worked perfectly before, and has only started to work terribly since two days ago.