It was not clear to me that whether you know French language or not. If you know the language, so spend some time to read the publication. May be the topic is so similar to yours, but it has a different solution or methodology than the one you have in mind.
A. Change your thesis topic
Talk to your advisor to see whether you can change your thesis topic or not. There may be some regulations in your university which may allow you to change your research topic in such case that happened to you.
B. Work on this topic
You can work on this topic but try to develop a new method to solve the problem, work on other questions related to your topic, apply existing methods you have seen in the literature to solve your question.
If somebody else has worked on the same topic does not really mean that you should not. As an example, many scientists have tried to solve the system of equations of motion, but they have different methods to do so, some go for numerical solutions and some other go for analytical methods and applied mathematics.
So if you think that you still want to work on the same topic, here are some of my suggestions:
Review the literature related to your topic carefully;
- Read that similar thesis you have found and the references to which it is referred. If it is in a language which you do not know, you can ask for help from a friend who knows the language or at least take a look at the equations, it's abstract and summaries, graphs, tables and etc.; also, the authors may have published some journal or conference papers after the submission of the thesis. Search for their name in the internet and take a look at their publications. These publications may be in English and help you get familiar with their work.
- Try to find books and papers which have worked on the same topic;
- Try to find solutions to similar problems to yours, think about whether you can apply their methodology to your own question or not;
Consult your advisor about how you can apply the new methods you have found to your problem. He may advise you that some methods may not lead to a solution and some other have chances to solve it. He may also encourage you to read some papers which you have not seen or found in literature.
Here are some warnings about your case which come to my mind;
Do not be upset and control your emotions; you are a researcher and research is not always an easy process. Do not become too angry or too upset. If you think you have problems on this, it will be a good idea to consult a doctor about it.
Please do not think that the solution or methodology which is presented in that thesis is the only way to solve the problem, free your mind and precisely search and think for newer and innovative solutions;
Do not loose time, as a student you may not have unlimited amount of time to spend on your research; based on the regulations of your university you may have a year or two to submit your final report;
- If you feel that you have to change your topic, consult your advisor, find a better topic and change your topic;
- If you think that you found a newer methodology to solve your problem, work on it, but if you see that you are wasting your time on this newer method, do not stop; consult with your advisor, look for another solution, read more papers; don't stop, move forward.