My Windows xp monitor became blue and blank.
I think I misclicked something in properties. I think my monitor became the 2nd monitor? Or something like that. Because I accidentally put a check on the "Use this device as primary monitor" on the number 2? Please help me.

So long as your not getting the bsod can you press ctrl/alt/delete and have anything show on the monitor? Does your mouse drag over to it?

Dumb question, but have you rebooted? Do you have a start button at least?

Also you should be able to make the first monitor the primary again if you Click Start, and then click Control Panel. Click Appearance and Themes, and then click Display. Choose which monitor you want as the primary, and then check the box for Use this display as my primary monitor.