1) Your system is loading the photos however due to the lack of system resources, it sort of hanged there... try restarting or lessen the load (reducing you applications)
2) If you are viewing it over an external hard disk which happend to me.... sometimes its the hard disk not responding normally..... re plug in again to prompt the computer to scan the disk again for the files.
3) you have recently moved the folders or files which caused LR not to be above to detect to image itself. If this is the prob, right click the file and use the locate file option to "guide lightroom 2" to the appropriate location. (happend to me b4)
For scenarios 1 & 2, i suggest you should restart your system and see if the loading can be completed. if not, copy the files to another location and import the files from there again.
Lastly, if the above still does not work, you might have corrupted files and re importing from your camera is you best bet.