Grey folder blinking with question mark when starting my MacBookPro?

Boot the MBP with the COMMAND + R keys.

It should result in a 4 option menu.

Click on Disk Utility.

Run Disk Utility>First Aid, Verify and Repair.

Any success?

Ciao.

“Cleaning up” your Mac is something that isn’t required with two exceptions. The Mac OS isn’t anything like Windows which does benefit from cleaning up due to its reliance on the Registry to track dynamic files and resources. Mac cleaning utilities clean your wallet but otherwise serve no purpose. The Mac OS automatically performs cleaning tasks regularly.

The exceptions are:

1. If your hard drive gets too full - Apple recommends that you keep about 15-20GB empty so that the OS and applications have room to create temporary files. If your drive gets too full find files to delete - data files such as music and video are good candidates since they tend to be big and deleting them won’t hard your computer. Deleting files from the Library and System folders is definitely not for the causal Mac user.
2. Sometimes a temporary file gets damaged and the regularly cleaning schedule cannot deal with it. Restarting your computer in Safe Mode (restart and hold the shift key until you see the Apple icon) will deal with this. After getting to the desktop restart normally.

For now you have only one choice: restart while holding Command R and then download and reinstall the operating system. You deleted one or more important files that the OS cannot live without.