Only a couple of general thoughts
Multiple faults at the same time are pretty rare, so I'd have a good look at the coil you have just installed - recheck connections (particularly earths), is it the right one for the system etc. Then I'd check the cheapest things first:
Check the kill switch
Change the plug
If you changed the plug cap along with the coil, swap them
If you can find another XL600 locally with a friendly owner, swap bits one at a time until it sparks again
If not get a haynes manual (if they do one) and follow their fault finding section
Good luck
