When the HP LaserJet 500 mfp m575 goes into powersave mode (sleep) the control panel is darkened to save power. The fact that you can print implies the device is still operable. When the control panel is dark and you send a print job to the device, does the control panel illuminate (it should)?
In addition to checking the electrical connections, which it sounds like you have done, please ensure the MFP is running the most current version of firmware. (You can get there via http://www.hp.com/, select Support > Download drivers. Enter the product name. From the produc page select "Firmware/BIOS upgrade," the "Latest firmware versions.") Here's a direct link: http://h20566.www2.hp.com/portal/site/hpsc/template.PAGE/public/psi/upgradeMigrateDisplay/?sp4ts.oid=5118241&spf_p.tpst=psiContentDisplay&spf_p.prp_psiContentDisplay=wsrp-navigationalState%3DdocId%253Demr_na-c03933242-12%257CdocLocale%253Den_US&javax.portlet.begCacheTok=com.vignette.cachetoken&javax.portlet.endCacheTok=com.vignette.cachetoken.