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Hello, my name is ***** ***** I'll be happy to assist you with this. It would appear that you have a bad hard drive. That would explain the "creeping problems" over the last few weeks and the missing partition. The chances are that you will not be able to access the drive to recover your data since it appears to have gone all the way out now. But you can remove the drive and hook it up to another computer as a usb external drive and see if you can access it that way.
As for this PC, you should purchase and install a new hard drive before putting the windows back on it. If you need any assistance with that let me know. Here's a link to an adapter that will allow you to hook your hard drive up to another PC.