Get a flashing folder with a question mark

This means that your computer is unable to find an operating system that it can load. Have you done anything recently which may have caused this? If not, you may have a failing hard drive. You could first restart in the Diagnostics Mode to get a report. Press D at startup. Then after any needed repairs get done, try to restart normally. If that fails, then I suggest restarting in the Recovery Mode. Press the Command + R keys at startup. This should enable you to download and install the computer's original operating system (which may be different than you have been running). If none of these things work, it is likely that your disk has failed. How old is it?