To find your 401k plan contact your former employer's HR department. Or you can see whether you have any old benefits information from your previous company. eg. your yearly or quarterly statements. Or finding someone from the company who have the same plan also will help.
Do you have a statement from them? If so call with your account number and see if they can track down what happened. Also, check with your plan administrator at work. They may have switched brokers, or made a change to the plan that caused it to be moved with JP Morgan.