Many variables:
1. You've had an accident - if it was your fault, your no-claims bonus has reduced (assuming you had any), and this coupled with the cost of the accident will increase your premium.
2. If it wasn't your fault, statistics show (apparently) that if you are involved in a non-fault accident, you're more likely to cause an accident within the next few months
3. Is your new car newer than the one that was written off, a higher spec model, etc - this will have an affect on your premium
This is obvously not considering whether you've got any convictions, where you live, age, etc as all these factors will play a part in the premium.