My iMac is not starting

For a vague question get a vague answer: What to do if your Mac doesn't start up.

For a more personalized suggestions can you tell us:

• Which iMac model do you have.
• What version of MacOS is installed.
• When your computer last worked normally.
• How have you already tried to solve your problem.
• And for 'not starting up' questions: Where in the startup sequence does your iMac fail. Is it a black brick? Do you hear a chime? Does anything show up on the screen?

See Writing an Effective Question for more tips.

You can use ANY USB keyboard, please restart in Safe Mode by holding down the Shift Key on restart until the computer starts up. NOTE: A Safe Mode restart can take up to 10 minutes so continue holding down the Shift Key.

Also, your post does not provide any useful information other than the model year. Providing statements such as "it has the latest version...." tells us nothing at all. Please be specific, something like " It has Catalina 10.15.3 installed on it" is specific.

Please carefully read Writing an effective Apple Support Commun… - Apple Community and then repost your question. Also, please refer to If your Mac won't turn on or start up and do all the steps there.

iMac won’t start up