THEATERGOERS got the shock of their lives when they showed up recently at Morgan Fairchild’s new play – the sexy star stood in the lobby, tearfully telling them the show wouldn’t go on.

Fairchild made the dramatic appearances after one of her female co-stars abruptly quit the cast of “High Infidelity” – forcing producers to cancel two performances.

As patrons entered the Promenade Theater at Broadway and 76th Street for yesterday’s matinee, they were approached by the blonde bombshell who said: “I’m so sorry, but we can’t do the show for you this afternoon.”

Then she begged them to stick around and see the evening show, featuring an understudy.

It was a gamble that paid off.

Most were so moved that the curvy actress had gone out of her way to explain things, they agreed to come back later.

“These things happen, and as they say, ‘There’s no business like show business,'” said Anne Laddin of Old Bridge, N.J.

Now in previews, the comedy – about a senator forced into marriage counseling by party bosses trying to save his sinking presidential bid – was shut down Tuesday night.

Hours earlier, co-star J.C. Wendel – who played the pivotal role of a nurse – walked out because of “artistic differences,” show sources said.

That left Lizby Larson, the understudy for both Wendel and Fairchild, scrambling to fill in.

Producer Jennifer Smith Rockwood said that while Larson was fully prepared for Fairchild’s part, she still needed work to take over for Wendel.

Fairchild made her lobby appeals Tuesday night and yesterday afternoon.

“Having been an audience member myself, I wanted to explain the situation and apologize for what happened,” Fairchild told The Post.

The play, which also stars John Davidson, will be in previews until Aug. 3, when it opens a regular run.