A manager asked an employee to perform a one hour task, three hours later he noticed that the employee was playing on his PC, Manager's reaction?

a manager has to know the reason first then take a reaction

First asked about the task completion if task done then evaluate the task and share with executives.

Second if not done then make completion of task with own supervison from another OR done slef.After completion of all work issue showcause notice to concern and final decesion will be make after & in the light of receiving of showcause notic reply.

check the task first,if it is not completed then go for the Disciplinary action,but if it is completed then suggest the employee to do more work instead of playing in office hours and offcourse dont give the employee more relaxed after his completion of work.

If this is the first time an employee can be routed oral warning him with careful follow-up and re-hosted and it was a duplicate administrative action must be taken against the employee through labor regulations

If the employee is done with the task, the Manager should first evaluate it and look for cons that the employee should should has spent time on instead of playing. If the task was done well, the Manager should might just let off with a Verbal warning.

As a Manager, we have to think from  both perspective:

First Chek the task completed or not? 

Second if its completed then it's my responsibililty to assign new task and utilize man power effectively.

It's also my responsibility to give an employee sufficient time for using an Internet or playing games to reduce Stress or Work pressure.

Create Win-Win Situation always.

My first reaction would be, How did we get here? Have you delegated the right task to the right person meaningfully or have you delegated the blame? did you follow up to check progress half way through the task? As you may know managers delegate never abdicate responsibility, the final decision will be subject to missing information. 

Check completion of task, evaluate quality of completed task, if task completed good or above, than suggest the employee to ask for more tasks to perform instead of playing.

If task is not completed, verbal warning (three warning -> out).

Than review the bonuses structure in order to check if there is enough money on the table to motivate employees to work hard in the direction of more results = more money

First he should know if he had finish the mission, if not he should be punished, because he has no excuse to play on PC, if he had any obstacles in his mission he should tell his manager about it to have some help.

always blame your self first..... train them on tme mgt...and teamwork..etc

Save the anger for later but right that moment ask him about the task you assigned.