If you have a written offer letter which you accepted, then it is a contract. If the employer breached that contract, you may well have a potential lawsuit. If the employer unilaterally changed the terms of the contract after you started, then they may or may not be permitted to do that, depending on what the contract says about how it can be modified.
You'll probably want to have an employment attorney review the contract with you to discuss your options.
Other potential claims you have are unpaid wages if money is still owed to you; fraud if you were lured into the position based upon false representations; and whistleblower claims if you were fired for objecting to the failure to pay wages owed.
I would suggest contacting an employment attorney to discuss any potential claims you have.