Open task manager and right click explorer.exe>End process>End process. Now, File>Run
type explorer.exe. If solved good, no, move on to next step.
If this has started happening recently, you can try system restore to.
System Restore
If this also doesn't work, run a system file scan. It could be that some system files might have been tampered with.
SFC /SCANNOW Command - System File Checker