My guess is that you have one of the following hosting related problems:
- Your host has a very low memory or maximum execution time limit for PHP. You need at least 24Mb of memory limit and 20 seconds of execution time to comfortably restore a site.
- The server has a maximum query per hour limit which was exceeded, killing the restoration script.
- The server has a CPU or I/O usage quota. Since the restoration is a CPU and disk intensive operation you exceeded your quota and the restoration was killed.
I think you'll have to contact your host and ask them if any of these is the case.
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joomfreak