Oh dont panic, problem solved!
I plugged camera to computer via usb cable, camera powered on, and started CHDK
without problem!
Glad to hear you got this sorted, but it's pretty strange. Removing the card should really should eliminate any possible impact from CHDK. The camera should boot without a card as long as the door is closed.
This might indicate some kind of hardware issue. Since it happened after switching lock, it could maybe be something like the switch that detects the battery door not engaging as srsa_4c suggests, or maybe something in the battery contacts.
Some models like SX10 seem to have a pattern of the SD lock detect switch failing, but this generally just causes the camera to see all cards as unlocked.