The most common reason that people cannot open surveys is that the survey and/or collector is closed or otherwise that it has expired. You can see if the survey is open by going to the Collectors page and looking at whether the large buttons show open or closed.
You might also have an automatic cut-off date/time set on your collector. Please see the article Setting a cut off time/date for information on how to set/edit and remove this setting. Note that if this is the case, the survey will only be closed when the first visitor clicks the link after the date/time has expired.
There are a number of reasons this might happen, that are listed below: