Mens rea is a concept that is one of many elements that must be proved for particular crimes and torts. The mens rea necessary for one offense and the mens rea necessary for another offense are often different.
For example, the mens rea requirement for murder and the mens rea necessary to hold someone liable for wrongful death in tort, are different.
If someone is not found to have had the necessary mens rea for a particular offense or tort, that person is absolved of criminal consequences, or civil liability, as to that particular offense or tort, but not as to offenses or torts with a less strict mens rea requirement.
Thus, if you negligently, but not intentionally or recklessly kill someone while driving your car, you are not guilty of the crime of murder, but do have civil liability in a wrongful death action for the death that you caused.
When there are multiple criminal offenses that are identical except for the mens rea requirement, with the higher mens rea requirement invariably associated with authorizing a more severe punishment, then the offenses with lower mens rea requirements are called "lesser included offenses" and depending on some rather arcane questions of criminal procedure, a jury may be asked to considering the most serious offense charged and also lesser included offense, at the same time as they deliberate. The jury could acquit as to an offense with a higher level of mens rea while convicting on an offense with a lower level of mens rea when that happens, if that is what they determine was the factual reality of the criminal defendant's intent to commit the crime.
But, it is also possible that a criminal defendant will be charged only with one offense, and will not be charged with lesser included offenses, and lesser included offenses will not be presented to a jury.
For example, there could be a situation in which it would make sense that someone has a intent to kill someone, and it would make sense that someone acted negligently in a manner that was not criminally negligent, but not intermediate fact patterns would be plausible under the circumstances. So, the criminal defendant might face only a murder charge and not other charges, and the jury would have to make an all or nothing call on criminal liability. A prosecutor might refrain from bringing lesser included charges to prevent the jury from being tempted to make a split the difference compromise verdict that is unlikely to reflect the factual reality.