Why won't my Mac recognize my keyboard and mouse pad, and why does it keep turning off?

We cannot provide any help unless at least tell us what iMac you own, meaning the display size and model year, the version of Mac OS installed, what troubleshooting you have tried, when the problem began and any other useful information that may help us troubleshoot your problem.

I strongly recommend that you look over the information in the following article:

Writing an effective Apple Support Commun… - Apple Community

Please download, run and post the resulting report created by Etrecheck according to this user tip: How to use the Add Text Feature When Posting an Etrecheck Report - Apple Community.