The source of the line
The line was originally spoken in a television commercial for a medical service called Lifecall, in 1990. The idea behind Lifecall was that many senior citizens today live at home alone, and may find themselves suddenly in medical danger with no one near to help them. Thus, Lifecall, a small radio badge worn around the wrist or neck; when the button on the badge is pressed, the senior is immediately put into contact with a dispatcher who can send a paramedic, fireman, or other emergency assistance.
The TV advertisement specifically featured a fictional elderly lady named "Mrs. Fletcher" who has fallen, with her walker, in the bathroom. Sprawled helplessly, she speaks the famous line, and a dispatcher informers her that 'help is being sent immediately'. On its face, the commercial illustrates a serious situation for a senior with dire consequences (elderly people with no one to care for them may fall in their homes and be on the floor for hours or even days, unable to get help). The Lifecall ad, however, had the double misfortune of being unintentionally campy - and being shown often on cable and daytime television. The combination made 'I've fallen...and I can't get up!' a recognized, universal punchline that applied to any number of comedic situations.
Lifecall for the United States has since gone out of business, although it may still exist in Canada by the same name. A number of other medical emergency services continue to provide help to senior citizens and disabled persons. According to business records, the largest company that provides this kind of service is Lifeline.
The actor
Mrs. Fletcher was played in the commercial by one Dorothy McHugh. The commercial's "Mrs. Fletcher" and her catchphrase became so famous that when Dorothy McHugh died on July 19, 1995, her death was newsworthy based on her appearance in the Lifecall commercial alone.
Notable uses
Jay Leno used this catchphrase after a motorcycle accident he had on September 14, 1991. He came back to The Tonight Show on crutches, and got one of the biggest laughs of the evening with the line. Saddam Hussein was rumored to have said it as well after his defeat in the Gulf War.