Thanks so much for responding. While I was waiting for an answer, I read suggestions on several sites, and some suggested PRAM / NVRAM reset and/or an SMC reset. So, I tried both, and nothing changed. Prior to those 2 things, I had run Disk First Aid and Repaired Disk and Permissions, Then, for lack of anything else to try, I shut the computer down, waited 30 minutes and did a cold boot, and the Wifi magically appeared. The computer has been running hot lately which concerns me, so maybe the cold boot did more than anything and the problem will come back?
On my computer, it always acknowledged that there was an Airport card installed - just that it was "not turned on", so it might be different from those who said it showed as "no card".
Anyway, I have no idea why it finally worked. A little history: I got a new battery about a month ago, (wouldn't hold a charge) but I think it was running hot before that. A while before that, I got a new charger, but I tried my old charger with my husband's computer and it didn't work with his either, (and his charger with my computer) so I am pretty sure both the battery and the charger were just old and needed to be replaced.
I would appreciate some advice. Occasionally, the screen scrambles on the laptop screen but not on the big screen I have attached. Sometimes it unscrambles when I click on something and sometimes it freezes up. I am wondering if this combined with my other problems is an indication that the logic board is about to go? If so, would it be worth it to pay for a diagnostic and possibly repair it, or should I just go ahead and get a new one? I have certainly gotten my money's worth. I'm a graphic designer/photographer, and I have used this computer all day, every day since May of 2007 (9 full years). I am at max RAM for this computer (6 GB - 667MHz), and I have a small hard drive (125 GB), so I would love a new faster and bigger computer, but I would rather not spend the money if I could get by with a repair.
Any thoughts? and thanks so much for your help with the original question!
Mary