It's good practice to set a spending limit on your Facebook ad account to prevent yourself from accidentally going over it. 

However, we often forget that limit. When that happens, the ads will no longer be spent and Facebook will not show them to your audience.

Follow these steps to update the spend limit:

1.Go into Payment Settings https://business.facebook.com/billing_hub/payment_settings
2.Account Spend Limit
3.Click on 3 dots to Update it or Delete it.