revert your network adapter driver back to an older version instead of the one installed by windows 8.1.
You can do this by going to device manager, right click on your network adapter, go to update driver, then "browse my computer for driver software, then"let me pick from a list of...." , uncheck the "show compatible hardware" checkbox and finally select the older version of current driver (if you dont know which, just use trial and error).
don't uncheck "show compatible". You see 2 drivers for the same broadcom adapter. One by Broadcom, the other from Microsoft. Choose the Broadcom one. Install. Springs back to life. (turning off heuristics,rebooting router,manual ip assignment none work)
I have the Lenovo G580, which has the Broadcom 802.11n Network Adaptor.
By the time I was having problems (don't know about previously) Windows was configured to use the Microsoft driver for this device.
Same fix as others have mentioned but I will provide a bit more detail. Basically I located a Broadcom-supplied driver on my computer, and updated to use that. Immediately wi-fi connected, and all seems well.
Here's the instructions:
# Open Device Manager (search Windows Help if you don't know what this is)
# Select 'Network adaptors' and then open (double-click) Broadcom 802.11n Network Adaptor
# Go to the Driver tab and click the Update Driver... button
# Select 'Browse my computer for driver software'
# Select 'Let me pick from a list of device drivers on my computer'
# Select the "Broadcom 802.11n Network Adaptor (Broadcom)" entry from the list, and click Next