You may have malware. Just about the only sensible thing you can do in such a case is use a "Rescue Disk".
You need access to another computer to create one. There are various systems but this one works quite well; Avira AntiVir Rescue System - Download
You need to create the disk, and then boot your machine using that disk.
Of course, you could try various other things like repair install, or even a new install, but in view of your description of what happened it is probably malware that has caused the problem, and a rescue disk might solve it for you without too much trouble.