DIAGNOSIS Yeah, this one's common. The printer is stuck in its startup or self-maintenance cycle, often because the print head is parked or it's trying to initialize a cartridge that it doesn't like.
FIX First, just turn it off at the power switch and unplug it from the wall for a full two minutes. This forces a hard reset. While it's off, open the front cover and check that the print head carriage moves freely—give it a gentle nudge if it's stuck. Plug it back in, turn it on, and let it go through its full, noisy startup dance. Don't touch any buttons until the 'wait' message clears.
IF THAT DIDN'T WORK The 'wait' message can also get stuck if there's a problem with a cartridge chip. Try removing and firmly re-seating all the ink cartridges. Make sure they're the Brother INKvestment cartridges (the big tanks) and that you've peeled off the clear protective tape from the chip.
HEADS UP If you interrupt the startup cycle by opening a cover or pressing buttons while it says 'wait', you'll often make it start over and wait longer. Patience is key with this step.