first n foremost just to be completely sure....you have tried the last config that worked option right.
if you have then the second option is to try n boot in safe mode....
if that doesnt try booting via the windows cd...go into safe mode....and go to system restore option...try for a date before you had the problem......
if you are unable to access safe mode....reboot ensure bios set to boot from the windows cd....go to repair option....its not a complete or new install..u wont lose anything...promise....second.....navigate to cmd line...use chkdsk -r option to repair......there are other options....such as fix boot....if there any issues with the boot files....but lets try one thing at a time.....
if this doesnt work post another entry....
Knowledge is just wasted time if not put to action
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Network +, A+, CCNA, CCDA, CXFS, Avaya (ACIS), MCP ...next stop CCNP,MCSE, CCNA Security, CCNA Voice, VCP