Ace Frehley -- Hospitalized ... Forced to Cancel Show

Ace Frehley will not be back, back in a New York groove ... instead he's laid up in a hospital bed.

The former KISS guitarist says he was admitted to a Pennsylvania hospital for exhaustion and dehydration ... after a concert Friday night.

Frehley is still hospitalized, and as a result had to cancel his scheduled show Saturday nigh in NYC, his hometown.