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
AmigaOS3: Amiga 1200
AmigaOS4: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad