Try this first

It could be jammed by hard things such as broken metal e.g. screws or hard bone piece. 

1. Unjam,

If disposal hums but doesn't turn, you need to find an allen wrench and unjam the disposal like in this video

https://www.youtube.com/watch?v=cdG520BhkNQ

Use brute force to unjam, wiggle allen wrench back and forth, it is made tough. You are unlikely to break it using only your arms' strength. 

2. Reset

If disposal does not hum, feel around in the bottom of the disposal to find a reset button and press it. 

Usually when the diposal jammed with switch on for more than a few seconds, it trips the safty switch inside to avoid overheating.  The reset button needs to be reset.  But after reset the disposal you still need to unjam the disposal as in step 1. 

3. Loud Noise

If you succeed but the disposal has a loud and/or high pitched noise, it means you've got hard items such as screws, broken metal etc down there that caused the original jamming.  You need to put your fingers into the disposal and feel around to locate the hard items.