How can I start my car, radio also not working. Engine cranks?

I have a Nissan Tino. Its been sitting for 3 weeks, and the battery went flat. I had to use pasenger manual lock to enter car, open bonnet, recharge battery. Now, car wont start and radio not working (it is not the original, the unit has been replaced by an aftermarket radio with usb). Both key fobs open and lock car, and when turning on, both have no effect on the first attempt, but both crank the engine on second and further attempts. I'm sure it is very simple solution - can anyone help?? Thanks in advance!!