It seems that your local user key has changed. Which version of QV / OS are you using? There are some reports in the discussions before that reported similar problems when changing network (office / home etc.)
You are entitled for up to 4 so called document recoveries in personal edition (the action vicky was probably using this morning), please read http://community.qlik.com/docs/DOC-1799 for further details.
But there was no change. I only have QV at work and I don't want to open files from forum. So, why has QV problems with opening. At least, QV has opened the document 2 hours ago without any problems.
Are the documents after the 4th time really unopenable? And what shall I do the next time?
I don't know why there is / was a problem with opening. I only know that Qlikview calculates somehow a user key and embeds this in every application file. If somehow your key has changed (don't ask me how, I don't know any details, just guessing that they might use different information pieces like OS version / MAC number etc to assemble one), you are asked if you want to use a document recovery.
You might use this up to 4 times (counting one time per key change, not file opening. Thus, if you have say 10 files with older key and you enable recovery for a new key use, all 10 files should open, resulting in a 1 time use of a recovery action).
After the 4th recovery, you are not able to recover any other files, those will be lost.
If this happens, you might have to upgrade to paid license for opening your own old files (new files are still accessible / you might still create new files).
This information is how I interprete the above linked official web site from qlikview (no file to download, just html). As I upgraded to a professional license some months ago, there is not much more to say from my side.
If you have any further problems, please post a little more information about OS / QV version, screenshot of problem dialog etc.