If you have a stuck mash -
Stuck mash; wort will not flow through the grain. to test this stop the pump and see if the wort level on top of the grain bed drops. if it does not move then this is a 'stuck mash'.
If it clears slowly then you have a 'slow mash' -
Give the grain bed a gentle stir with your paddle; consider adding 1-2 handfuls of rice or oat hulls if you don't have these on hand uncrushed barley malt will work but will add some colour and may add a small amount of fermentable to your wort. Look to decrease your grain crush on your next batch. it is likely caused by a large portion of the grain bill being powdery or too finer crush.
be wary that a slow mash will decrease efficiency, therefore you will want to increase your mash time to be on the safe side increase by 1/2 hour or do starch test to ensure mash has completed
If you don't fix a stuck mash it will likely lead to a stuck sparge.