If you reach your daily limit, all ad groups in the campaign are paused. Here are the details:
Daily - Accelerated or Shared - Accelerated: If you reach your daily limit before the end of the day, all ad groups in the campaign are paused (Budget paused status), and the ads in those ad groups are no longer displayed. If you don't want this to happen, it's a good idea to select either the Daily - Standard or the Shared - Standard option.
It's important to note that your daily limit (accelerated or standard) is a target -- your actual spend might be higher or lower. Variations are caused by a number of factors, such as different traffic volumes in different days of the week, or automatic detection and refunding of fraud clicks that can give money back to a campaign within a few hours of the click. Microsoft Advertising anticipates and automatically compensates for the fluctuations, and usually keeps overspend to less than 100% above your daily limit.
If you don't take action to resume the campaign in Budget paused status, the campaign automatically restarts at the beginning of the next month.
Your daily budget setting is only a target, so your actual spend might be higher or lower. This is because, regardless of the budget type you choose, Microsoft Advertising always calculates a monthly budget and then the actual budget limit is calculated for an entire month. Daily budget types simply help you make sure that your budget is spread out over an entire month, and not used up in the first day.
The calculation to convert your daily budget to a monthly budget is:
monthly budget = daily budget x days
If you happen to change your daily budget mid-month, then your current monthly limit is calculated as your spend so far that month, plus the daily budget amount multiplied by the number of days remaining in the month. Like this:
monthly budget = Spend-to-date + (daily budget x days remaining)
In the unlikely event that your campaign’s spend goes over your monthly limit, Microsoft Advertising will refund the overspend at the end of the month.
To learn more about choosing a budget type, take a look at: What are my budget options?