The workbench disks should not be giving you a guru.
But at least it seems your system is fine. Maybe the workbench disk is faulty or your floppy drive is damaged or dirty.
Did you try any other disks you may have to see if they load? Games, software, anything.
If they load then your floppy drive is fine and your workbench disk is not.
With regards to the accelerator, you could try pressing any socketed chips down as they may have come loose over time. You can also try cleaning the edge connector on the Amiga using a PCC (Printed circuit cleaner). You can use that to clean the slot of the accelerator too.
