I have some money in my account that I haven’t used in the past few days, but when I checked it a few days later, it turned into a negative value. Has my account been stolen?
Change your API key and make sure that you don’t accidentally expose it in your application.
Also, check the activity log (export file is available) in the “Usage” page. Be aware that if you use the OpenAI playground, this counts also against your account.
It’s probably been hacked. Or you use a leaked key on a public platform 
Can I just delete the old API key and create a new one to deal with this?
Yes, but there may be a delay in billing. 
That alone will not work; you need to discover what significant mistake you’ve made in keeping API keys secret and never exposed to client code or a code repository. They should never be given to third parties, put into websites, included with your code, used to have client software directly connect with OpenAI…
I’m currently battling hay fever and my vision is a bit blurry, so I may not be able to provide much advice. However, utilizing a tool like git-secrets could help you avoid accidental commits to git.