If this doesn’t work the first time, make sure you’ve run the ‘Startup Repair’ option first. Then go into the command prompt and run the commands. The startup repair will often fix the issue, but if it doesn’t, it will at least point the partition tables to the proper OS folder.
TZ,
Rarestia has given you more detailed instructions on using bootrec. I recommend you follow those their advice since I haven’t had to do this for quite some time.