You might have started a campaign (maybe for testing purposes) which you forgot to make inactive. This happened with me once, and I realised after a few days when I logged back to my Adwords account Luckily it was just $ 5.xx that were owed
i have a lot to say about this.. but .. lets not. all i can say is.. don't cancel the account, lower the bids if you contact support to cancel the account .. you will be banned for life i mean google's life as a company
lower the bids is not the correct to do for a bad conversion, but other things like reevaluation of the keywords, the ads and landing pages. If someone ask to cancel the account this person will not be banned from AdWords.
Yep... I cancel and reopen Adwords... no prob for me. As for the sudden activation of keywords. Maybe your CTR improve or it is really relavance so that the CPC decrease making inactive keywords active.