My key went flat igot a new oneit still wont start

I stopped at the shop and when I came out 5 mins later my key wouldn't unlock the doors or boot I opened the door with the key tried to start the car dashlights came on but it didn't even attempt to start assuming it was my key I tried to replace the battery but when I removed the battery I wiped its memory I got a new key cut and reprogramed it locked then unlocked the doors cranked like it was going to start then nothing went dead again and key wouldn't unlock doors security light on dash is off