Was it really a Mass if a woman celebrated it?
The short answer
Most likely what you attended was a Communion service, which lay people — including women — may lead when no priest is available. No consecration happens at such a service; the hosts were consecrated earlier at a Mass. If you saw a woman attempt to consecrate the bread and wine, report it to the diocesan bishop.
Probably a Communion service
Without more information it is impossible to be sure, but from what you were told, it sounds like a Communion service rather than a Mass.
What a Communion service is
Lay people, including laywomen, may lead Communion services when no priest is present. The Eucharist is not consecrated at these services, because only a priest can consecrate. The hosts distributed were consecrated beforehand at another Mass.
An example
On Holy Thursday the priest consecrates twice the usual number of hosts, because no Mass is offered on Good Friday; the extra hosts are reserved for the Good Friday Communion service.
If something was wrong
If you saw the woman attempting to consecrate the bread and wine, contact the bishop of that diocese and tell him what you witnessed.
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