Can bed rest cause someone to die when they try walking and cause heart to fail from no exercise?

Unlikely: If one restricts activity/exercise very much or long, the body loses it exercise tolerance and what was easy or normal before now seems to take a great deal of effort with heart pounding and puffing over very little activity. However with gradually increasing time and pace of activity, the body quickly regains the exercise tolerance to meet the demands. Inactivity causes disability but not death.