-Stammer-

#1 thing to so first is to backup your data on your hard drive ASAP (It *might* be the reason for your computer crapping out).

This is a stupid question, but do you know what your hard drive looks like?  If so, unplug both cables, and reinsert them (plus dust out as well).  It sounds like  the cable isn't seated properly, and may have come loose when you moved it.. If it is intermittently booting up, that may be a postive thing.  If the hard drive is failing, they often just flat out die, or they will make a really funny clicking noise.  That said, I would still backup your data as best you can, just to be safe.

Run this utility after backing up your important files (I recommend Google Drive, you get quite a lot of storage for free): https://www.howtogeek.com/134735/how-to-see-if-your-hard-drive-is-dying/

It will tell you if the hard drive is crapping out, you'll have to buy a new one if that's the case. Getting Windows back onto the machine will be a different beast all together. Depends on if you have a big box PC (dell, lenovo, gateway, etc) or if it was a custom built. EIther way you'll either need your manufacturers restore DVDs or your original discs for windows with the genuine license key.

If you don't have either of those, then you can purchase legit keys online for fairly cheap here: https://www.keyshoponline.com/

Buy a license, download (I recommend upgrading to Windows 10, Windows 7 is end of life) from here: https://www.microsoft.com/en-ca/software-download/windows10

Pay special attention to the key youre buying if going that route, and just get the matching ISO from Microsoft. Burn it to a USB, swap the hard drives and you should be good. You may have to adjust your BIOS settings to make sure it boots from USB, or look in your manual for the keyboard short cut to force the boot menu to pop up.

Good luck.