This means the drive is unable to read the stored data. Running a utility such as "DiskCheckup" to look at the "SMART" error counters will probably reveal "reallocated" or "pending" sectors.
The first thing you should do is to get a copy of as much data as you can from your drive in case it fails.
The next thing to do is to monitor the "reallocated" and "pending" sector counts. If those counts are increasing then your drive is probably failing, and it would be wise to replace it.