You say you had a contract of employment. If you did, and if it limited the grounds for termination (such as for theft), then you could, if you believe you did not do the things accused of, sue your employer for breach of contract. In your lawsuit, you would have to prove that you did not steal, or otherwise do something providing grounds for termination. You could use your own testimony, testimony of co-workers or other witnesses, any docuumentaty evidence, etc. If there was no actual contract, however, or if there was one but it did not limit the grounds for termination, then you would have no recourse, because in those cases, you may be fired at any time, for any reason.