Sometimes a quick reset can resolve a startup issue however If your Mac is stalling on the progress bar or you receive a grey screen with folder and a flashing question mark. Your connection cable from the hard drive to the system board is possibly faulty and not detecting the hard drive, a replacement cable should resolve this issue with no data loss.
Otherwise the internal hard drive is starting to fail or has failed completely.
The hard drive can be replaced and the OS.X (Apple operating system) reinstalled and we can have you up and running relatively quickly. Whether the data on the faulty drive can be retrieved will depend on the condition. Please note if you suspect there is a fault with the drive, refrain from continually turning the device on and off as this could be the key factor to successfully getting your data back.
Putting added pressure on a problematic drive may cause it fail completely thus reducing your chances of a successful recovery.