It's likely the immobilizer failing which is a theft deterrent system that prevents the car from starting if it thinks it's being stolen. Apparently, faulty immobilizers are a common problem in S40/V40s. You can also tell on the dashboard where in the lower right-hand corner, there is flashing car icon with an "X" over it. This icon means the Immobilizer has been activated and hence the car won't start. In the Volvo-forum website, someone suggested locking and unlocking the doors five times with the remote, which sends a reset signal to the immobilizer. I tried this several times myself, and it seems to cure the problem each time.