Fatal error: Cannot redeclare class K2HelperHTML in /Applications/MAMP/htdocs/instyle/Administrator/components/com_k2/helpers/html.php on line 14

Please mention which version of K2 and Joomla! you are using.
Also which other extension are you using and where is this issue appearing?

Im using joomla 3.34 stable version with local server php 5.5.10. Problem appears when i click on component, then k2. All of the drop down options dont work such as categories, items, users etc. and it come's up with a blank screen displaying that code.
As a far as extensions im using a gavick instyle template, offaljn vetical drop down, offaljin search bar.

Please try reinstalling K2's latest version.
Do not uninstall it because you will lose your data. Simply install it on top.