screen goes black after start up

[clay1053]

hello.... i am new to the mac world. i have a mac G5, running leopard 10.5 . i was recently fooling around with the resolution in the systems preference. i switched to a larger pixel and suddenly the screen went black with a vertical fine red line going down the right side of the screen. i've tried unpluging the tower, and restarting and tried unpluging the monitor and restarting. still the results are the same. when the computer starts, i get the apple logo and the little spinning disc beneath the apple, when that dissapears, the screen turns light blue, then goes completly black with the red vertical line. i am completly stumped. short of sending it off somewhere to be repaired, can you help.

clay