If the link goes directly to a thank you page or to a message saying you've already taken the survey, then the survey is set to allow only one response per browser. Please contact the survey creator who can change the settings to turn on Multiple Responses.
If you opened a survey to find responses already filled in, this is because the survey has Response Editing turned on AND Multiple Responses turned off. Please contact the survey creator who can turn off Response Editing.
If you're trying to access a survey and are taken to the SurveyMonkey homepage instead, this means the link you used was incorrect. Links are character and case sensitive, so an extra space or a lower-case letter in place of an upper-case letter could break the link.
Rather than clicking on the hyperlink to access the survey, try copying and pasting the URL into your browser's address bar. If that doesn't work, please contact the survey creator who can give you the correct link.