no internal speakers | digital output only | red light from audio jack

I plugged in a stereo through analogue mini jack to the headphone socket of my mac book pro. When i unplugged the stereo, the internal speakers failed to work. There is now a red light coming from the jack as if a "digital output" is plugged into the socket. But no digital output has ever been plugged into the socket - and I have no control of mute or volume settings.

When I plug headphones in, they work fine and I have control of mute and volume settings. Once the headphones are removed I am automatically set back to "digital output" with no control over mute or volume, and no audio from m internal speakers.

I have tried changing the settings in system preferences/sound/settings/output ... but there is no option other than "digital output". I suppose I would like to select "internal speakers" but there is no option for this. And like I stated earlier, the jack is lit up red.

I've tried software updates, restarts, disk utilities (permissions and repairs including repairing with the startup disk), plugging and unplugging, but to no avail.

When I try to change the volume on the keyboard's volume controls I get the "no entry" sign under the volume bar and have no control (as if it were stuck on mute).

The only thing I can think of is that I forced a mini jack into the headphone socket too hard so I'm worried that I've damaged the hardware.

Could this have caused the problem? Or is this a software issue? Any help would be appreciated.

In short I have no sound from the internal speakers.