There could be several reasons for this issue. First of all, make sure that you enter the correct e-mail and password. Use the e-mail address entered at signing up your Merchium account.
Another probable reason is that the User e-mail is used as login setting was disabled on the Settings > General page in the administration panel. In this case you should enter your sername instead of the e-mail to log in.
You can also use the Forgot your password? link to reset your password.