As said Amigaharry2, check if Open firmware visible with other cards, or try to boot from installation CD.
Maybe your 9250PCI is 64MB version and your new resolutions is simply too large...
If Open firmware is visible, try to boot to MorphOS bootmenu:
- press "shift" during boot process
- od add parameter "bm" to your boot command
in bootmenu press "v" to toggle resolution
AmigaOS: Pegasos II, Sam440ep-flex, Micro A1-C, AmigaOneX1000
MorphOS: Pegasos II, Powerbook G4, iMac G5, Powermac G5 Quad