Wall-E wrote:
Next time, make sure you turn the camera OFF before you change lenses.
If it doesn't fire right away, try taking the lens off [turning the camera off first] and putting it on a couple of times to clean the contacts.
Been using various Canon dSLRs for years. I have never bothered to turn the camera off while changing lenses and have never had a problem. Most pros and advanced amateurs do the same.