The pensioner, Miss Zona Wilson, 93, left a plastic plant pot on top of her electric cooker.
The hob was on at the time and began to melt the pot, sending smoke pouring into her smoke detector.
It triggered, startling her and causing her to fall and cut the back of her head.
The alarm also alerted neighbours at the Memorial Gardens sheltered accomodation, including neighbour Liz Newsham, who called 999.
Police and firefighters were sent to the scene.
Fire crews from Garstang and Fulwood administered first aid to the Miss Wilson before an ambulance took her to the Royal Preston Hospital, where she was treated before being released.
Gary West, watch manager at Fulwood fire station, said: "The lady has heard the smoke detector going off, fallen over and banged her head, suffering cuts to the back of her head."
* Anyone who would like a free home fire safety check can call 0800 169 1125.