It depends. Investors are buying shares in your company, which the company is selling. Their rights, and your obligations to them, are determined in the rights of the share class you are selling and the shareholders agreement.
If you are issuing non-voting common shares, the share type typically offered, then no you do not have any obligation to pay them back.
However, your investors are investing in the business under the assumption that the business will make money and provide them returns in one way or another. Read about different types of return here.
Speak to your lawyer or email alexander.morsink@equivesto.com to learn more about the implication of different rights for shareholders.