There are two ways Jane feels trapped at Thornfield. First, in a famous passage, Jane thinks about how quiet and stifling her life as a governess is. Mr. Rochester has not yet arrived, and Jane is stuck, unable to leave, with only the housekeeper and her pupil, the young child Adele, for company. Like many women of her time period, she simply doesn't have enough to do. She has many talents and passions and nowhere to exercise them. While she knows she should be grateful for her easy job and life, at the same time she is bored half to death and longs for more, for some adventure or challenge. As she says to herself:
It is in vain to say human beings ought to be satisfied with tranquillity: they must have action; and they will make it if they cannot find it. Millions are condemned to a stiller doom than mine, and millions are in silent revolt against their lot. Nobody knows how many rebellions besides political rebellions ferment in the masses of life which people earth. Women are supposed to be very calm generally: but women feel just as men feel; they need exercise for their faculties, and a field for their efforts, as much as their brothers do; they suffer from too rigid a restraint, to absolute a stagnation, precisely as men would suffer.
Later, Jane feels trapped when she finds out that Mr. Rochester had planned to deceive her and pretend to marry her while he was already married to Bertha. Much as she loves him, she believes she must flee immediately, and she does.