There are times when MakerBots build plates get stuck. This is a lack of lubrication on the downrod that moves the plate up and down - big screw along the backside of the printer viewable from the front.
The same issue exists when your MakerBot makes a loud grinding noise
To resolve the issue: Put on a rubber glove, put a glob of lube on your fingers and apply it to the downrod.
Lube should solve the problem.