Check and ensure that power cord is plugged in. If the power cord is plugged into a working power outlet and there is food inside the workbowl, try removing some of the contents and attempt use again. Failure to "turn on" may be caused by overloading the workbowl to a point where the contents prevent the motor from functioning properly.
This is a common complaint among user reviews at websites like Amazon. To achieve maximum uniformity in the size of the processed food pieces, pre-cut/chop food on a cutting board into smaller pieces. Attempts to process food that has already been properly prepared for the food processor will yield more desirable results than attempting to process whole food items.
In this case, one of a few things may have occurred: (1) The blade mechanism may have become disengaged with the motor. (1.1) This may require replacing the blade mechanism and/or the motor. (1) The workbowl may be have too much contents, thus overloading the power of the motor (1.1) remove some contents in the workbowl and attempt normal use. Discontinue and repeat until proper function is attained. If proper function does not occur, repair may be necessary. (1) The motor itself may be damaged internally. (1.1) The motor needs replacement.