I think of two possible reasons:
1. you've got a 68060 accelerator and try to boot from a floopy with the original 68040.library on it, so SetPatch crashes immediately
2. your floppy disk is infected with a bootblock virus which does not run on your Amiga and crashes.
For the first case you should make a copy of the disk and delete 68040.library from the copy, then try to boot again.
For the second case you should turn off your computer, then boot from HDD and install a new bootblock to the disk (open a shell window and enter install df0:).
Bye,
Thomas
