Good morning/evening

Does anyone know what I should do about my medieval 2 total war; it won't let me play online because it says the key is already being used by someone else

I have bought it no downloads etc and the key was entered at installation correctly I checked o's and 0's but still can't play online

It seams someone else has my key and I can't play online can anyone help and if anyone wants a battle online just ask
Cheers