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Changing-look quasars are a recently identified class of active galaxies inwhich the strong UV continuum and/or broad optical hydrogen emission linesassociated with unobscured quasars either appear or disappear on timescales ofmonths to years. The physical processes responsible for this behaviour arestill debated, but changes in the black hole accretion rate or accretion diskstructure appear more likely than changes in obscuration. Here we report onfour epochs of spectroscopy of SDSS J110057.70-005304.5, a quasar at a redshiftof $z=0.378$ whose UV continuum and broad hydrogen emission lines have faded,and then returned over the past $\approx$20 years. The change in this quasarwas initially identified in the infrared, and an archival spectrum from 2010shows an intermediate phase of the transition during which the flux belowrest-frame $\approx$3400\AA\ has decreased by close to an order of magnitude.This combination is unique compared to previously published examples ofchanging-look quasars, and is best explained by dramatic changes in theinnermost regions of the accretion disk. The optical continuum has been risingsince mid-2016, leading to a prediction of a rise in hydrogen emission lineflux in the next year. Increases in the infrared flux are beginning to follow,delayed by a $\sim$3 year observed timescale. If our model is confirmed, thephysics of changing-look quasars are governed by processes at the innermoststable circular orbit (ISCO) around the black hole, and the structure of theinnermost disk. The easily identifiable and monitored changing-look quasarswould then provide a new probe and laboratory of the nuclear central engine.

A new physical interpretation of optical and infrared variability in quasars