@omarsantana , omar , I recommend not fixing it yourself, not impossible, incorrect placement or uneven amounts of glue on one side or dry unevenly, image viewed by the mirror may be a bit distorted. Canon may fix it for free as there was sort of a recall(not positive as it is older). See 1st link below.
If wanting to give it a try yourself the other links may help in reinstalling the mirror. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.
https://petapixel.com/2015/05/13/canon-warns-the-original-5d-may-suffer-from-mirror-separation/
https://planesandstuff.wordpress.com/2014/08/16/diy-canon-eos-5d-camera-fix/
http://www.4photos.de/camera-diy/Canon_5D_mirror_replacement.html
https://www.lifepixel.com/tutorials/infrared-diy-tutorials/canon-5d