Once one has learned how to obtain and maintain jhana can the ability be lost (i.e., forgotten), due to lack of use.
One good example from the Buddha’s time was Devadatta, who was a brother of princess Yasodhara. Devadatta became a monk and developed the mundane jhanas and attained those direct knowledges described above. He could perform many “miracles”, and one time he appeared in the bedroom of Prince Ajasattu to impress him. But when Devadatta went against the Buddha and at one time injured the Buddha, he lost all his mundane powers and ended up in the lowest realm (avici niraya) because of those offenses.
There is this story about a yogi who was travelling by air with abhinna powers and saw a flower in the shape of a woman (called “närilathä”) and lost the jhanic state and came down; there is another such story where the yogi heard the singing of a woman and had to face the same fate.
Yes you can. Forgetfulness is very common. Everyday I lose/forget jhana throughout the day and everyday I have to wake up and remember again. It is a daily, moment to moment practice to remember the jhana.
Here is a story from Zen Flesh, Zen Bones.
Zen students are with their masters at least two years before they presume to teach others. Nan-in was visited by Tenno, who, having passed his apprenticeship, had become a teacher. The day happened to be rainy, so Tenno wore wooden clogs and carried an umbrella. After greeting him Nan-in remarked: “I suppose you left your wooden clogs in the vestibule. I want to know if your umbrella is on the right or left side of the clogs.”
Tenno, confused, had no instant answer. He realized that he was unable to carry his Zen every minute. He became Nan-in’s pupil, and he studied six more years to accomplish his every-minute Zen.
First of all, You can switch the Jhana to turn on and off.
Sexual intercourse and Alcohol might reduce the concentration for a while tho.
Once you become a stream-enterer, it's not possible to lose your Ability to get into Jhana.
Breaking the Precepts will help you lose your ability tho because a lot of hindrances will popup and hinder your concentration.