I was staking BCC and it said there are only 2 days left until a staking reward.  Then suddenly my computer accidently restarted and now I open the wallet and unlock and the it says "Not staking because you don't have mature coins".

So this means that I have to wait the whole 15 days again to get a reward just because my computer restarted 2 days before a reward?

I read in the FAQ that the computer does not need to be turned on during the full 15 days.  Only during the last day or so.