Ok so I have windows Vista. My monitor wouldn't turn on, so I had to send it in to get fixed. Instead, they sent me a new monitor. Pretty sure it's an HP 2009m...or something like that.

Now my problem is, webpages aren't displayed properly. For example, most colours are non-existent now, some tabs are invisible and I have to hover over them to find them etc.
To use a more specific example, I frequently go to www.mmaspot.net. If you go there, you'll notice that the website is mostly red, black and gray. It used to be like that for me as well. However, now it's just white, with missing tabs. I asked on their forums if anybody knew my computer problem, and unfortunately nobody could help. I found this forum, and I'm hoping it can be the solution to all of my problems

I tried to go to control panel > display > change resolution and all of those options, and NOTHING worked.
HELP PLEASE!Welcome to CH.
We are ready to help.
A question, Have you spoke to the PEOPLE that did the job?
They should have given you a CD with information about how to set up the new monitor. If so, did you try the install CD they gave you with the new monitor.
Can you please check and see what is the name of the monitor?

In Vista you can open the Device Manager and see if the new monitor is installed.
Screen Short of Device manager in Vista:
http://waxjar.com/devices.jpg
In my system it shows generic devises. But for the monitor you have it needs to be the specific monitor, a special driver is needed.
Well, I didn't talk to the people who did the job because when the new monitor was ready, I went and PICKED it up and installed it myself. I didn't mess with any of the details, I just plugged it in and connected it.

I followed your advice and checked the device manager, and it said that the monitor is properly installed.
My monitor is an HP 2009m 20" LCD.Quote

My monitor is an HP 2009m 20" LCD.