On LR 3, whenever a crash occurs, it's automatically generated an "lock" file. I'm not pretty sure whether this is also true for LR 4, but it worths trying: go into your LR folder, look for a file whose name has "lock" in it, and delete it. Be careful not to delete your root file, though, or you would loose all the changes made on the photos...
The "lock" file is actually named "your catalog name".lrcat.lock.
As far as I know, this has been true on every version of LR and it is also true of LR5 beta.
The file is created to try to disallow two computers from opening the same catalog at the one time. It is not completely fool-proof, but you must be extremely lucky to defeat it.