Hello there! Is mac not turning on at all ( no lights) or are you not able to get passed the grey screen?
if you cannot get passed the grey screen please follow the steps below
Reboot into the Recovery HD by choosing the ‘Apple‘ menu > Restart, hold down the ‘Command’ + R’ keys on the keyboard during the start up process when you see the Apple Logo you can let go. When the machine boots eventually you will see the OS X Utilities screen., select the Disk Utility and then select Continue. Then in Disk Utility select Macintosh HD in the left column, if there are two or it is named something else choose the first indented item, and then select Verify Disk on the right. If that is OK in green type then select Repair Permissions. If it is not ok let me know. If that is ok quit Disk Utility and Reinstall
What do you see on the screen?
Hi there and welcome
It seems your last expert has opted out.
When you at disk utility, at the top left you see Verify Disk, can you click this please?
Hi, please let me know when I can call you