Your imac could have a power supply or mainboard issue if it gets hot easily, unable to power on one day, failed to power up after a power trip or if your house kept having power trip after turning on the iMac.
Cracked screen, vertical stripes, thin pink colored screen, vertical lines, or flickering screen monitor. At times, the whole screen is black or with no backlight. The only way is to replace the iMac LCD
Your iMac gets slower to start up & launch into the operating system. At times, it hangs there or gets longer to launch a program. This happens when the hard drive gets old and the only way is to replace to a new HDD/SSD