Q: Had to shut down mac computer as it froze and now when trying to start has a grey screen with a folder and question mark in it flashing is this something to worry about

You appreciate in the iPad forum. You should either be asking in the iMac or Mountain Lion forum. I have asked the hosts to move your post to somewhere more appropriate.

I assume your iMac is Intel.

https://discussions.apple.com/community/desktop_computers/imac_intel

Here is the Mountain Lion forum in case you need it.

https://discussions.apple.com/community/mac_os/os_x_mountain_lion

Check this Knowledge-base article for possible help:

http://support.apple.com/kb/TS1440?viewlocale=en_US&locale=en_US

Restart the computer while hold down the command and R keys. Release the keys after you see the spinning wheel.

On the next screen select the recovery drive and hit return.

On the next screen select Disk Utility and continue.

In the list at the left, select the hard drive.(Be sure to select an item that’s indented to the right in the list, not an item at the far left.)

Click First Aid.

Click Repair Disk.

When that is done, quit DU and restart in the usual way.