Though uncommon, your available spend could end up in the negative. This most often happens when you have used balance on your account and Brex updates your limit to below that of your used balance.
The best way to address this is to follow these steps.
Make an early payment to free up spending room. Early payments will clear in 3-5 business days. Follow instructions noted in How do I make a payment to Brex?
Determine why your limit was dropped or zeroed. Something may require your attention. Read more details in $0 Brex Credit Limits