During a project iteration, half of the project team was reallocated to a new project.

Raise the risk of having the velocity impacted for the current iteration and discuss it with the product owner.

Raise the issue with the product owner to bring additional resources to the project team.

Raise the risk with the project stakeholders and wait until the velocity for the current iteration is calculated.

Raise the issue with the project stakeholders to analyze the project impact and take the proper actions.