Olimometer disappeared from website

I’ve seen this recently on other WordPress installations, but not specifically for my plugin. It has usually been because the site has been compromised and malicious code injected into various php files. The malicious code can cause the plugin to become invalid and WordPress won’t show it despite all of the files still being there. You should install Wordfence and “Anti-Malware and Brute-Force Security by ELI” plugins and do a scan of your site. If it finds anything, clean it up, change your passwords, then to fix the Olimometer just delete the whole folder from wp-content/plugins and reinstall.