Why would the power only to the upstairs lights go off mysteriously?
The loss of electrical power described in the question may be symptomatic of potentially serious issues that might result in fire and/or injurious or fatal electri shock. It needs prompt attention from a person competent to analyze and work on residential electrical systems. Hiring a licensed electrician is strongly recommended as a general recommendation.

The interruption of power to a portion of your home was undoubtedly caused by an absence of electricity in the conductors serving that portion of the building. Normally this is caused by a open circuit.

It is easy to talk in theory. However, a house that was built in 1890 was certainly built without electrical service. Such service was added later. Because there are both circuit breakers and fuses in the ceiling, it is highly likely that several renovations/additions to the electrical service have been made since it was installed.

Therefore, the only way to determine the pertinent details is by physically observing the actual installation.