Sudden trouble with airport

Hi guys we have an imac and a macbook pro connected to internet with airport, this had been working great until yesterday when for some reason we couldn't connect either computer.
We are not very experienced with apple or all the tech talk but could someone please help? i have no idea why it would suddenly stop working, sometimes we get a very weak signal on both but can't load pages properly or receive mail.
Sometimes the base station is recognized sometimes not any ideas in simple terms appreciated.

Thanks