Cell c expires airtime and data

Good morning, I have a real problem with expiring pre paid airtime for data and phone calls. This month I had enough airtime to last the whole month but due to me not recharging in time I lost all my airtime now I can't make calls or check messages. I had airtime for this now its gone. This in my eyes amounts to theft. I know what the terms and conditions say but it makes no sense to take away my airtime which I have paid for now I can't make calls or connect to the internet. I had enough airtime now its gone again. Please explain why airtime must expire, I want to load airtime for as long as possible when I have money and not have to worry about it being ******.