Why don't you have a backup on an external HDD?
I don't do Windows, so my trouble shooting ability is limited for that platform but this is what I would do. First set the OS to not go to sleep. Then quit iTunes and restart the computer. Launch iTunes and start downloading the music in more managable chunks.
I also strongly suggest you buy an external HDD (ideally about 3:1 in size) and start backing up your personal data.
I back up to a number of cloud services but none of them will give me the uncorrupted library that I am hoping to get from iTunes Match, not to mention that I paid to have this capability.
Bear in mind that iTunes Match is not a backup for your music. Apple does not advertise the service as such and it should not be depended on to be an exact backup like the data on the external HDD was. For more information read this User Tip: FAQ: Why iTunes Match can NOT be used as a backup!
I hope you're recovering from the flood!