Sounds like a channel conflicting problem or interference from your neighbors
Your likely going to need a professional come by with sniffer software and sort the mess out, get your router channels away from thiers or recommend a more powerful wireless N or something.
Until then use a Ethernet cable to connect the Mac to the router or modem, if the modem, then you have to power on/off while connected to the Macto get the ISP to honor it.
I personally figured it out on why this problem occur, well let me just hit to the point.
What happens is that the router of the wifi channel is being changed automatically or someone might accidentally changed it. MBP can't detect wifi from certain wireless channel. So, we have to change the channel of the router wifi channel that may be detectable by your MBP.
THE SOLUTION -
1. Connect to the wifi from a active device to the router wifi that your MB can't detect or use Ethernet cable to get connected.
2.Once connected, access to the router wifi menu by entering the ip address on your net browser, eg:192.168.2.1
3.Then go to wifi setup or menu, and you may come across the wireless channel.
4.Switch the channel from any numeric to numeric that the router wifi can make your MB detect it.
eg: Wireless Channel - 6 (change it to -->) Wireless Channel - 11
5.Save the settings for the router wifi, exit, restart your MB and HAVE FUN!