My computer can't seem to recognize my monitor anymore

I have a Dell Inpsiron 545S, only a month old.  

Yesterday I spent two hours on the phone with Dell, to discover that the constant rebooting, was caused by a faulty memory card.  

My problem now, is that all my display settings were reset.  Now that the faulty memory has been removed, I've been trying to get my display settings and screen resolution back to the way it was, but for some reason the computer no longer recognizes my Acer X193W monitor, like it did before.  Now it tells me that I have a Generic Non-PnP Monitor.  The trouble I'm having is that it won't let me select the 1440x900 resolution that I had before.  Now I'm only given three choices of 800x600 ; 1024x768 ; 1280x720

How can I get my computer to recognize the monitor that it had no trouble detecting before..??  I've tried downloading a driver update for my Acer monitor, but when I try to update the driver, it tells me that I'm already using an up to date driver.

Help.. :emotion-7: