There are many explanations for that.
1) The test is wrong (very unlikely, but it happens). Ask the company for clarifications.
2) Your mother had an affair with another man (unlikely, but it's the case of about 1% of us).
3) You're adopted and your parents haven't told it to you yet (unlikely, but again, it is possible and you have the right to know it).
4) Maybe one or both of your parents was infertile and they had to use an anonymous sperm/egg donor.
Are you a guy? Ask your father to do a paternity test. If he's your biological dad, it is expected that you two carry the same haplotypes in your Y chromosomes. Ask also your mother to do a mtDNA test. If your mtDNA is different, then she's unlikey to be your biological mother.
Wht are your blood types? Certain outcomes are not possible. For example, if your dad is B and your mom is O, you can't be A. If your dad is A and your mom is A, you can't be B. I know the case of a girl that found out that she wasn't the biological daughter of his father because their blood types didn't match.
Be careful with what you decide to do next. Truth hurts sometimes.