DIAGNOSIS This is usually a programming lockout. The machine thinks it's in 'program' mode for a future brew, so brew button is disabled.
FIX First, check if the 'PROG' light is on. If it is, press and hold the 'PROG' button for about 3 seconds until the light turns off. That should clear the scheduled brew and give you back control. Then just hit the 'ON/OFF' button to start a regular brew cycle.
IF THAT DIDN'T WORK Try a full reset. Unplug the machine, wait a full minute, then plug it back in. This often clears any temporary electronic glitches that can cause the controls to freeze up.
HEADS UP It's easy to accidentally bump the 'PROG' button when cleaning the control panel. If this keeps happening, just get in the habit of checking for that little light before you walk away thinking it's broken.