Reason for that problem is bootmgr file in c: drive is deleted by u r mistake or virus delete that.
Solution: step 1:u need ubuntu or any linux cd a pendrive.
step 2:u not need above step1 but virus can affect other system hdd.
for step 2: remove hdd/ssd insert in other working system . goto c: drive remove tick in "hide all hidden files and windows files" now u see bootmgr file in c: drive copy and paste in u r os installed Hdd drive.
step 1:copy bootmgr file to pendrive from other system.boot from linux cd open c:drive paste the file.
restart.remove cd. now all fine.