External display settings Macbook...FRUSTRATING

So I have been using an external display with my Macbook, a Dell E207WFP. This has worked great for a while now; usually, I would just power up the laptop with the monitor connected and just hit "Detect Displays" from the menu bar icon. My monitor and Macbook screens would automatically adjust to the highest resolutions possible, and my resolution on the Dell would always register at 1680 x 1050.

All of a sudden, this seamless integration of external displays stopped working and every time I clicked on "Detect Displays", the Dell monitor registers at a default of 800 x 600. Worst of all, the option to display at 1680 x 1050 has disappeared and the highest resolution I can work with is 1400 x 1050. Does anybody know what's going on? Any possible solutions? I want to be able to return to the beautiful 1680 x 1050 resolution if possible. Thanks.

Specifications: Macbook 1.83 GHz Intel Core Duo, 2 GB 667 MHz SDRAM, OS X Leopard 10.5.4.

Yes, it shows the resolutions of the laptop screen and the Dell external monitor...the laptop screen is at its maximum of 1280 x 800 and the Dell resolution is 1400 x 1050. That's the same thing that shows up when I go to System Preferences ---> Displays.

Just to make sure it's not the monitor, I plugged in my Windows laptop to the monitor and got the 1680 x 1050 resolution perfectly. On the Macbook, it's the weirdest thing, the 1680 x 1050 doesn't even show up in the list of display resolutions in System Preferences. It just disappeared...