When messing around with different video refresh rates, you could end up with a garbled screen full of unreadable junk. To bring the video_server back to a sane state, hit Ctrl+Alt+Shift+F12. BeOS will reset the video to 640x480x8bpp.
In extreme cases, you can also fix things manually by moving drivers around. This should seldom be necessary as the above should always work, but just in case, here’s the scoop:
Reboot into the fail-safe graphics mode and delete the appropriate graphics-card driver in /boot/beos/system/add-ons/app_server, and the corresponding accelerant in /boot/beos/system/add-ons/accelerants. Reboot, and the resolution will be the default 640×480 8bpp. You should now be able to reset your resolution and color depth from the Screen preferences panel.