Is the illness that befell the questioner related to his secretly taking money from employers who exploited him, or is it a trial from Allah, and what must he do to please Allah Almighty?

If you both agreed upon the wage and working hours, and the employer paid you what was agreed upon, then what you took is not rightfully yours and is considered treachery. It must be returned to its owner, even if indirectly, because the agreement is binding, and "Muslims are bound by their conditions." If you feel wronged, confront your employer or look for another job, but it is not permissible to take his money under the pretext of injustice. However, if the employer violated the contract and did not give you your rights, then you are permitted to take your right without exceeding limits. As for illness, it may be a punishment for sins or a trial, and one should not claim to be free of sins.