Did ASDM work ok before, on the same workstation?
Does it still work ok from another workstation?
Did you change anything on the now failing workstation? E.g. upgrade JAVA?
What messages do you get in the Java console?
BTW a reboot is not needed to replace the ASDM image. I don't think there is anything wrong with the image though. You might be running into a bug (without the Java console output it is hard to say) so you may want to try a newer version of ASDM...