Hi,

Are you able to see cloudera manager with the VM you are using for prcaticing?
If yes, go to the web-url of cloudera manager and locate JobTracker daemon and select re-start option. Same for Task Tracker.

If no cloudera manager is available, direct command to start JT and TT is as follows:[any of these should help, try in same order]
Command1: start mapred.sh
Command2: hadoop jobtracker

Hope it helps