Lost the plot
British SlangDefinition
To become confused or irrational, losing grasp of what’s happening or how to cope.
Examples
"After an all-nighter of revision, I completely lost the plot in the exam and forgot my own postcode."
"When the gaffer started rearranging desks at 3 a.m., we realised he’d lost the plot."
"With the timetables changing every hour, the whole station had lost the plot by lunchtime."