If the unit has been dropped - the speaker will separate since it is only made of high density plastic of some sort.
If this isn't a warranty problem, find the proper bits for your screwdriver, take the grill plate off, then remove the speaker (driver). Check to see that the speaker isn't broken. I have had this happen twice - once under warranty, the last one was on me. I found that If I glued the parts back together with JB Weld (tough stuff) clamp together for the required time and , I was lucky.
I love this little amp for my guitar and voice at small venues.