This invention relates to a process for preparing a stable low viscosity polymer polyol having a solids content of at least about 30% by weight. This process comprise free-radically polymerizing one or more base polyols, one or more pre-formed stabilizers, and one or more ethylenically unsaturated monomers in the presence of one or more free-radical polymerization initiators, and one or more polymer control agents; in which the process is restarted after being stopped by (I) restarting the feed of base polyol and the initiator; (II) continuing the base polyol and initiator feed until the total feed amounts to at least 5% of the total reactor volume; and (III) restarting the feed of the ethylenically unsaturated monomers in which the ratio of monomers to total feed is ramped up to its final value over a period of not less than 25% of the reactor residence time. This process is particularly suitable for restarting the process of production after any planned or unplanned reactor shutdown.