If box 2 (Federal Income Tax withheld) is blank you should immediately contact your employer. There are two things that could have happened that would make your W-2 correct.
1.) Your earnings were so low that no tax was needed.
2.) You submitted a Form W-4 stating you were exempt from income tax.
Other than that there is no reason not to have income tax withheld, and your employer, or payroll department should be able to give you an answer.
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