Since you've not made a Statutory Off Road Notification, you can't tax your car from 1st April; you'll have to pay for the tax from 1st March.
Unless your car is subject to SORN, you are now obliged to ensure that there's a valid insurance policy in place for it at all times (even if it's in the garage). So, technically, you've committed an offence. However, as long as you get the tax and insurance sorted out as soon as possible, you've absolutely nothing to worry about.
Chris