Normally when the back/next buttons stop working it is because the survey design put in custom validation into the survey which is causing a javascript error.
Remove the custom validation or required validation, secondary validation and primary validation and the error will most likely go away.
Check your code that you are inputting, fix it and try again.
This can be due to control ID's changing because of a missing ClientID tag set to AutoID in the web.config.
As usual the normal fix would be to upgrade the application - use the new web.config and make sure you are using the correct .net framework.