Ok, so this issue started for me Sunday. Up until then as far as I can tell my MBP late 2009 was connecting to my wireless Comcast router with no problems.
Sunday after church I open up my computer and no connection?? However, my Iphone 5S is connected and my wife's dell is connected no problem.
I decide to change to my Verizon MIFI Jetpack, connects no problem. I then start to search the message boards for similar problems and try several suggested things.
Long story short I change the wifi encryption on the comcast modem to WPA2 and problem solved.