1. Identify whether its under scope or not ?

2. If its with in the scope, then there is a mistake on effort/cost estimation or in the planning.

3. Here the case.. we should delivery with in the budget or estimated cost.

4. Revise the plan and make it strictly.

5. When you revise the plan, you will identify the resource who is lead and lag.

6. Have meeting with the team mates and let them know the situation and increase the work time.

7. According to ur revise plan ( allocate the idle resource and lead resource ) start doing the work.

8. Make sure the daily review and work towards the revised plan which you made against your requirement ( Delivery with in the cost is the purpose).