One possibility is that you moved the catalog when you transferred your photos to the external drive. Or you moved it by other means.
The messages that you get are an indication that LR doesn't find the catalog it used to work with.
You have to find the catalog LR worked with when it last opened.
The catalog is a file with the ending <.lrcat>.
See here where it is normally found in various OS: http://kb2.adobe.com/cps/843/cpsid_84313.html
When you find this file double-click it and LR should open with it.
Thank you so much for replying. I have looked into my external drive and found a few folders which are Adobe folders. They include some CS4 (InDesign, Illustrator, Dreamweaver, etc.)and Lightroom folders. I opened them to see what's in there and it appears that one of them contain the <lrcat> file. Instead of going through all the folders, I was thinking that I should just drag each folder out of the external drive and just drop them back into the My Pictures folder on my computer. Will that make my Lightroom accessible to me again?
Thank you for this helpful information! I ran into the same problem and realized I had deleted the catalog when I moved files out of lightroom.
Now, however, Lightroom is saying that the catalog is corrupted, and that it is unable to be read. I have tried clicking "repair" but it keeps telling me it won't work and the file can't be found. Any suggestions?