Is your critical path defined as a number other than zero? If defined as a negative number there may well be no critical path (according to that definition).
Try the following:
1. Ensure you have enabled float path calculation [Tools>Schedule>Options>Advanced tab>check option to "Calculate multiple float paths"]
2. Group the project schedule by Float Paths and Sort by Remaining Early Start
3. Look for the float path with the least amount of float that also has a "meaningful" chain of activities that extend from the data date to project completion. In other words, ignore (or rectify as needed) float paths that are too short and have just a handful of activities. This meaningful path with the least amount of float should be your most critical path.
Hope this helps!