Why My Megane 2003 Dynamique wont start?

My Megane will not start, the key card battery has been replaced and the engine battery. I went for a short drive & when I returned to my car it wouldn't start. The heating came on though. I tried the lock & unlock the door and pointing the keys to the immobiliser sensor as per advice given, but it has just happened randomly. The key card insert doesn't appear loose & I do get the press clutch/brake to start message. How much will it cost to replace a keycard reader? I only have one key.