If you exceed the spend limit, your automated and post-boosting rules as well as reports will continue working but you wouldn't be able to edit them or create new ones till the start of the next billing period. Once the next billing cycle starts your monthly cumulative spend will start over again. 

If it is a matter of some urgency you can always upgrade to a plan of a higher notch at any time and not have to wait for the beginning of the new month.

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